emersion from the egg differ considerably from the parent, especially in having only the thoracic portion of the body as yet evolved, and in possessing but a small number of locomotive appendages; the visual organs, too, are frequently wanting at first. (See Fig. 276, C-G.) The process of development, however, takes place with great rapidity; the animal at each successive moult (which process is very commonly repeated at intervals of a day or two) presenting some new parts, and becoming more and more like its parent, which it very early resembles in its power of multiplication, the female laying eggs before she has attained her own full size. Even when the Entomostraca have attained their full growth, they continue to exuviate their shell at short intervals during the whole of life; and the purpose which seems to be answered by this repeated moulting, is the preventing the animal from being injured, or its movements obstructed, by the overgrowth of parasitic Animalcules and Confervæ; weak and sickly individuals being frequently seen to be so covered with such parasites, that their motion and life are soon arrested, apparently because they have not strength to cast off and renew their envelopes. The process of development appears to depend in some degree upon the influence of light, being retarded when the animals are secluded from it; but its rate is still more influenced by heat; and this appears also to be the chief agent that regulates the time which elapses between the moultings of the adult, these, as in the Daphnia, taking place at intervals of two days in warm summer weather, whilst several days intervene between them when the weather is colder. The cast shell carries with it the sheaths not only of the limbs and plumes, but of the most delicate hairs and setæ which are attached to them. If the animal have previously sustained the loss of a member, it is generally renewed at the next moult, as in higher Crustacea.1

372. Closely connected with the Entomostracous group is the tribe of Suctorial Crustacea; which for the most part live as parasites upon the exterior of other animals (especially Fish), whose juices they imbibe by means of the peculiar proboscislike organ which takes in them the place of the jaws of other Crustaceans; whilst other appendages, representing the feetjaws, are furnished with hooks, by which these parasites attach themselves to the animals from whose juices they derive their nutriment. Many of the Suctorial Crustacea bear a strong resemblance, even in their adult condition, to certain Entomostraca; but more commonly it is between the earlier forms of the two groups that the resemblance is the closest, most of the Suctoria undergoing such extraordinary changes in their progress towards the adult condition, that if their complete forms were alone attended to, they might be excluded from the class alto

1 For a complete and detailed account of this group, see Dr. Baird's "Natural History of the British Entomostraca," published by the Ray Society.

gether, as has (in fact) been done by many Zoologists. Among those Suctorial Crustacea which present the nearest approach to the ordinary Entomostracous type, may be specially mentioned the Argulus foliaceus, which attaches itself to the surface of the bodies of fresh-water fish, and is commonly known under the name of the "fish-louse." This animal has his body covered with a large firm oval shield, which does not extend, however, over the posterior part of the abdomen. The mouth is armed with a pair of styliform mandibles; and on each side of the proboscis there is a large short cylindrical appendage, terminated by a curious sort of sucking disk, with another pair of longer jointed members, terminated by prehensile hooks. These two pairs of appendages, which are probably to be considered as representing the feet-jaws, are followed by four pairs of legs, which, like those of the Branchiopoda, are chiefly adapted for swimming; and the tail, also, is a kind of swimmeret. This little animal can leave the fish on which it feeds, and then swims freely in the water, usually in a straight line, but frequently and suddenly changing its direction, and sometimes turning over and over several times in succession. The stomach is remarkable for the large cæcal prolongations which it sends out on either side, immediately beneath the shell; for these subdivide and ramify in such a manner, that they are distributed almost as minutely as the cæcal prolongations of the stomach of the Planaria (Fig. 273). The proper alimentary canal, however, is continued backwards from the central cavity of the stomach, as an intestinal tube, which terminates in an anal orifice at the extremity of the abdomen.1

373. From the parasitic suctorial Crustacea, the transition is not really so abrupt as it might at first sight appear, to the class of Cirrhipeda, consisting of the Barnacles and their allies; which, like many of the Suctoria, are fixed to one spot during the adult portion of their lives, but come into the world in a condition that bears a strong resemblance to the early state of many of the Crustacea. The departure from the ordinary Crustacean type in the adult, is, in fact, so great, that it is not surprising that Zoologists in general should have separated them; their superficial resemblance to the Mollusca, indeed, having caused most systematists to rank them in that series, until due weight was given to those structural features which mark their Articulated charac

ter. We must limit ourselves in our notice of this group, to that very remarkable part of their history, the Microscopic study of which has contributed most essentially to the elucidation of their real nature. The observations of Mr. J. V. Thompson, with the extensions and rectifications which they have subsequently re

As this group is rather interesting to the professed Naturalist than to the amateur Microscopist, even an outline view of it would be unsuitable to the present treatise: and the Anthor would refer such of his readers as may desire to study it, to the admirable treatise by Dr. Baird, already referred to.

2 Zoological Researches," No. III, 1830.

ceived from others, show that there is no essential difference between the early forms of the sessile (Balanidæ, or "acorn-shells") and of the pedunculated Cirrhipeds (Lepadide or "barnacles"); for that both are active little animals (Fig. 277, A), possessing three pairs of legs and a pair of compound eyes, and having the body covered with an expanded shield, like that of many Entomostracous Crustaceans, so as in no essential particular to differ from the larva of Cyclops (Fig. 276, c). After going through a series of metamorphoses, one stage of which is represented in Fig. 277, B, C, these larvæ come to present a form D, which reminds us strongly of that of Daphnia; the body being enclosed

FIG. 277.

с

Development of Balanus balanoides :-A, earliest form:-B, larva after second moult ;-0, side view of the same;-D, stage immediately preceding the loss of activity; a, stomach (?); b, nucleus of future attachment (?).

in a shell composed of two valves, which are united along the back, whilst they are free along their lower margin, where they separate for the protrusion of a large and strong anterior pair of prehensile limbs provided with an adhesive sucker and hooks, and of six pairs of posterior legs adapted for swimming. This bivalve shell, with the members of both kinds, is subsequently thrown off; the animal then attaches itself by its head, a portion of which becomes excessively elongated into the "peduncle" of the Barnacle, whilst in Balanus it expands into a broad disk of adhesion; the first thoracic segment sends backwards a prolongation, which arches over the rest of the body so as completely to enclose it, and of which the exterior layer is consolidated into the "multivalve" shell; whilst from the other thoracic segments are evolved the six pairs of cirrhi,-which are long, slender, many-jointed, tendril-like appendages, fringed with delicate filaments covered with cilia, whose action serves both to bring food

to the mouth, and to maintain aerating currents in the water,— from whose peculiar character the name of the group is derived. 374. The chief points of interest to the Microscopist in the more highly organized forms of Crustacea, are furnished by the structure of the shell, and by the metamorphosis of the larva, both of which may be best studied in the commonest kinds. The shell of the Decapods in its most complete form consists of three strata; namely, 1, a horny structureless layer covering the exterior; 2, a cellular stratum; and 3, a laminated tubular substance. The innermost and even the middle layers, however, may be altogether wanting; thus in the Phyllosoma or "glasscrabs," the envelope is formed by the transparent horny layer alone; and in many of the small Crabs belonging to the genus Portuna, the whole substance of the carapace beneath the horny investment is made up of hexagonal thick-walled cells. (It may be here noticed, that the carapace of Daphnia, Branchipus, and some other Entomostraca, exhibits the hexagonal division; whilst in many species this is not distinguishable.). It is in the large thick-shelled Crabs that we find the three layers most differentiated. Thus in the common Cancer pagurus, we may easily separate the structureless horny covering after a short maceration in dilute acid; the cellular layer, in which the pigmentary matter of the colored parts of the shell is contained, may be easily brought into view by grinding away as flat a piece as can be selected, from the inner side, having first cemented the outer surface to the glass slide, and by examining this with a magnifying power of 250 diameters, driving a strong light through it with the achromatic condenser; whilst the tubular structure of the thick inner layer may be readily demonstrated, by means of sections parallel and perpendicular to its surface. This structure, which very strongly resembles that of dentine (§ 406), save that the tubuli do not branch, but remain of the same size through their whole course, may be particularly well seen in the black extremity of the claw, which (apparently from some difference in the molecular arrangement of the mineral particles, the organic structure being precisely the same) is much denser than the rest of the shell, the former having almost the semitransparency of ivory, whilst the latter has a chalky opacity. In a transverse section of the claw, the tubuli may be seen to radiate from the central cavity towards the surface, so as very strongly to resemble their arrangement in a tooth; and the resemblance is still further increased by the presence, at tolerably regular intervals, of minute sinuosities corresponding with the laminations of the shell, which seem, like the "secondary curvatures" of the dentinal tubuli, to indicate successive stages in the calcification of the animal basis. This inner layer rises up through the pigmentary layer of the Crab's shell, in little papillary elevations; and it is from the deficiency of the pigmentary layer at these parts, that the colored portion of the shell derives its

minutely speckled appearance. Many departures from this type are presented by the different species of Decapods; thus in the Prawns there are large stellate pigment-cells (resembling those of Fig. 327, c), the colors of which are often in remarkable conformity with those of the bottom of the rock-pools frequented by these creatures; whilst in the Shrimps there is seldom any distinct trace of the cellular layer, and the calcareous portion of the skeleton is disposed in the form of concentric rings, an approach to which arrangement is seen in the papillæ of the surface of the deepest layer of the Crab's shell.

375. It is a very curious circumstance, that a strongly marked difference exists between Crustaceans that are otherwise very closely allied, in regard to the degree of change to which their young are subject in their progress towards the adult condition. For whilst the common Crab, Lobster, Spiny Lobster, Prawn, and Shrimp, undergo a regular metamorphosis, the young of the Land Crab and the Cray-fish come forth from the egg in a form which corresponds in all essential particulars with that of their parents. Generally speaking, a strong resemblance exists among the young of all the species of Decapods which undergo a metamorphosis, whether they are afterwards to belong to the brachyourous (short-tailed) or to the macrourous (long-tailed) division of the group; and the forms of these larvæ are so peculiar, and so entirely different from any of those into which they are ultimately to be developed, that they were considered as belonging to a distinct genus, Zoea, until their real nature was first ascertained by Mr. J. V. Thompson. Thus in the earliest state of Carcinus menas (small edible crab), we see the head and thorax, which form the principal bulk of the body, included within a large cafapace or shield (Fig. 278, a) furnished with a long projecting spine, beneath which the fin-feet are put forth; whilst the abdominal

B

FIG. 278.

D

Metamorphosis of Carcinus mænas:-A, first stage; B, second stage; c, third stage, in which it begins to assume the adult form; D, perfect form.

segments, narrowed and prolonged, carry at the end a flattened tail-fin, by the strokes of which upon the water, the propulsion of the animal is chiefly effected. Its condition is hence compa