peculiarly fitted for Mr. Furze's combination of the polarizing plate with the spotted lens (§ 63); as they then exhibit all the richness of coloration which the former developes, with the peculiar solidity or appearance of projection which they derive from the use of the latter.

FIG. 225.

Thaumantias pilosella, one of the "naked-eyed" Medusa :-a a, oral tentacula; b, stomach; c, gastrovascular canals, having the ova ries, dd, on either side, and terminating in the marginal canal, e e.

306. No result of Microscopic research has been more unexpected, than the discovery of the close relationship subsisting between the Hydroid Zoophytes and the Medusoid Acalephæ (or jelly-fish). We have seen that many of the small free-swimming Medusans, belonging to that simple tribe of which Thaumantias (Fig. 225) may be taken as a representative, are really to be considered as the detached sexual apparatus of the Zoophytes from which they have been budded off, endowed with independent organs of nutrition and locomotion, whereby they become capable of maintaining their own existence and of developing their generative products. The general conformation of these organs will be understood from the accompanying figure. Many of this group are very beautiful objects for Microscopic examination, being small enough to be viewed entire in the zoophyte-trough. There are few parts of the coast on which they may not be found, especially on a calm warm day, by skimming the surface of the sea with a fine muslin net attached to a ring, which may either be fixed to the end of a stick held in the hand, or may be fastened by a string to the stern of the boat as a tow-net. In either case, the net should be taken up from time to time, held so as to allow the water it contains to drain through it, and then turned inside out (so that what was previously its internal surface shall now be the external), and moved about in a bucket of water, so that any minute animals adhering to it may be washed off. When we turn from these small and simple forms, to the large and highly-developed Medusans which are commonly known as "jelly-fish," we find that their history is essentially similar; for their progeny have been ascertained to develope themselves in the first instance under the polype form, and to lead a life which in all essential respects is zoophytic; their development into Medusæ taking place only in the closing phase of their existence, and then rather by gemmation from the original polype, than by a metamorphosis of its own fabric. The embryo emerges from the cavity of its parent, within which the first stages of its development have taken place, in the condition of a ciliated gemmule, of rather oblong form, very closely resembling an Infusory animalcule, but destitute of a mouth. One end soon contracts and attaches itself, however, so

as to form a foot; the other enlarges and opens to form a mouth, four tubercles sprouting around it, which grow into tentacula; whilst the central cells melt down to form the cavity of the stomach. Thus a Hydra-like polype is formed, which soon acquires many additional tentacula; and this, according to the observations of Sir J. G. Dalyell, leads in every important particular the life of a Hydra, propagates like it by repeated gemmation, so that whole colonies are formed as offsets from a single stock, and can be multiplied like it by artificial division, each segment developing itself into a perfect Hydra. There seems to be no definite limit to its continuance in this state, or to its power of giving origin to new polype buds; but under conditions not yet ascertained, the Strobila (as it is termed) ceases to propagate by ordinary gemmation, and enters upon an entirely new series of changes. In the first place, the body becomes more cylindrical in form than it previously was; then a constriction or indentation is seen around it, just below the ring which encircles the mouth and gives origin to the tentacula; and similar constrictions are soon repeated around the lower parts of the cylinder, so as to give to the whole body somewhat the appearance of a rouleau of coins; a sort of fleshy bulb, somewhat of the form of the original polype, being still left at the attached extremity (Fig. 226, A). The number of circles is

Successive Stages of Development of Medusa buds from Strobila larva :-a, polype body; b, its original circle of tentacula; c, its secondary circle of tentacula; d, proboscis of most advanced Medusa disk; e, polype bud from side of polype body.

indefinite, and all are not formed at once, new constrictions appearing below, after the upper portions have been detached; as many as 30 or even 40 have thus been produced in one specimen. The constrictions then gradually deepen, so as to divide the cylinder into a pile of saucer-like bodies; the division being most complete above, and the upper disks usually presenting some increase in their diameter: and whilst this is taking place, the edges of the disks become divided into lobes (B), each lobe soon presenting the cleft with the supposed rudimentary eye (more probably an auditory organ) at the bottom of it, which is

to be plainly seen in the detached Medusæ (Fig. 227, c). Up to this period, the tentacula of the original polype surmount the highest of the disks; but before the detachment of the topmost disk, this circle disappears, and a new one is developed at the summit of the bulb which remains at the base of the pile (c, c). At last, the topmost and largest disk begins to exhibit a sort of convulsive struggle; it becomes detached and swims freely away; and the same series of changes takes place from above downwards, until the whole pile of disks is detached and converted into free swimming Medusa. But the original polypoid body still remains; and may return to its polype-like and original mode of gemmation (D, e), becoming the progenitor of a new colony of Strobila, every one of which may in its turn bud off a pile of Medusa disks.

307. The bodies thus detached have all the essential characters of the adult Medusa. Each consists of an umbrella-like disk, divided at its edge into a variable number of lobes, usually eight; and of a stomach, which occupies a considerable proportion of the disk, and projects downwards in the form of a proboscis, in the centre of which is the quadrangular mouth (Fig. 227, A, B). As the animal advances towards maturity, the in

FIG. 227.

D

B

Development of Medusa from the detached gemmæ of Strobila :-A, individual viewed sideways, and enlarged, showing the proboscis, a, and b the bifid lobes; B, individual seen from above, showing the bifid lobes of the margin, and the quadrilateral mouth; c, one of the bifid lobes still more enlarged, showing the ocellus (?) at the bottom of the cleft; D, group of young Medusæ as seen swimming in the water, of the natural size.

tervals between the segments of the border of the disk gradually fill up, so that the divisions are obliterated; tubular prolongations of the stomach extend themselves over the disk; and from its borders there sprout forth tendril-like filaments, which

hang down like a fringe around its margin. From the four angles of the mouth, which, even in the youngest detached animal, admits of being greatly extended and protruded, prolongations are put forth, which form the four large tentacula of the adult. And finally the generative organs make their appearance in four chambers disposed around the stomach; which are occupied by plaited membranous ribands, containing spermcells in the male, and ova in the female; and the embryoes evolved from the latter, when they have been fertilized by the agency of the former, repeat the extraordinary cycle of phenomena which has been now described.

308. In connection with the preceding, it will be convenient to mention two curious little marine animals of frequent occurrence, the true place of which in the scale it seems difficult to determine, but which, having the free swimming habits and the soft texture of the Medusa, have been very commonly ranked as members of the same class. One of these is the Cydippe pileus (Fig. 228, A) very commonly known as the Beroë, which

FIG. 228.

A, Cydippe pileus with its tentacles extended; B, Beroë Forskalii, showing the tubular prolongations of the stomach.

designation, however, properly appertains to another animal (B) of the same grade of organization. The body of Cydippe is a nearly globular mass of soft jelly, usually about three-eighths of an inch in diameter; and it may be observed, even with the naked eye, to be marked by eight bright bands, which proceed from pole to pole like meridian lines. These bands are seen with the microscope to be formed of rows of large cilia, which are in a state of pretty constant vibration, though sometimes they are at rest; and if the sunlight should fall upon them when they are in activity, they display very beautiful iridescent colors. The mouth of the animal, situated at one of the poles, leads to a stomachal cavity of cylindrical shape, which extends about as far as the centre of the body, and then narrows into an intestinal tube which terminates at the opposite pole; from this stomach

FIG. 229.

tubular prolongations pass off beneath the ciliated bands, very much as in the true Beroë (B). In addition to the rows of cilia, the Cydippe is furnished with a pair of locomotive organs of a very peculiar kind; these are long tendril-like filaments, arising from the bottom of a pair of cavities in the posterior part of the body, and furnished with lateral branches (A); within these cavities they are often doubled up, so as not to be visible externally; and when they are ejected, which often happens quite suddenly, the main filaments first come forth, and the lateral tendrils subsequently uncoil themselves, to be drawn in again and packed up within the cavities, with almost equal suddenness. The liveliness of this little creature, which may sometimes be collected in large quantities at once by the muslin net, renders it a most beautiful subject for observation when due scope is given to its movements; but for the sake of microscopic examination, it is of course necessary to confine these. Various species of true Beroë, some of them even attaining the size of a small lemon, are occasionally to be met with on our coasts; in all of which the movements of the body are effected by the like agency of cilia, arranged in meridional bands. Very dif ferent, however, is the structure of another little globular jellylike animal, the Noctiluca miliaris (Fig. 229), to which the diffused luminosity of the sea, a beautiful phenomenon that is of very frequent occurrence on our shores, is chiefly attributable. This animal is just large enough to be discerned by the naked eye, when the water in which it may be swimming is contained in a glass jar exposed to the light; and a tail-like appendage, marked with transverse rings, which is employed by the animal as an instrument of locomotion, both for swimming and for pushing, may also be observed with a handglass. Near the point of its implantation in the body, is a definite mouth, on one side of which a projecting tooth has been seen by Mr. Huxley; and this mouth leads through a sort of œsophagus, into a large irregular cavity, apparently channelled out in the jelly-like substance of the body, and therefore considered by some in the light of a mere "vacuole," though by Mr. Huxley it is considered to possess regular walls; whilst from its cavity there passes forth a prolongation, which leads, in his belief, to a distinct anal orifice. The external coat is denser than the contained sarcode; and the former sends thread-like prolongations through the latter, so as to divide the entire body into irregular chambers, in some of which "vacuoles" are fre

Noctiluca miliaris.

1 "Quart. Journ. of Microsc. Science," vol. iii, p. 49; see also pp. 102, 199.