Page:EB1911 - Volume 23.djvu/802

Rh appendages have acquired a new and quite special development. They are six in number, median, ventral and dorsal, and two unequal lateral pairs. The largest is placed ventrally at some distance distal to the mouth. Its free extremity is a plumose fan-like expansion (fig. 5, Aa and H). It is, in common with others, a hollow process into which run two pairs of broad, coarsely transversely striated muscles. Each pair has a single insertion on the inner wall—the one pair near the free extremity of the limb, the other near its attachment; the bands run up, one of each pair on each side, and run right round the body forming an incomplete muscular girdle, the ends approximating in the median line. Above this point springs the large median dorsal limb, which terminates in groups of long setae. It presents a single pair of muscles attached along its inner wall which run up and form a muscular girdle round the body in its posterior third. On either side is attached a dorso-lateral and ventro-lateral appendage, each with a fan-like plumose termination consisting of compound hairs or setae, found elsewhere only among (q.v.); each of these is moved by muscles running upwards towards the neck and arising immediately under the trochal disk, the inferior ventro-lateral pair also presenting muscles which form a girdle in the hind region of the body. It bears a group of long setose hairs the bases of which are connected with the nerve fibre. There are also two pairs of distal antennae. Pedalion presents a pair of ciliated toes in the posterior region of the body (fig. 5, B, C, and D, e), which it can apparently use as a means of attachment; Dr Hudson states that he has seen it anchored by these and swimming round and round in a circle.

Reproduction Organs.—Rotifera are unisexual, with the sexes dimorphic. The ovary is, as in many Platyhelminthes, duplex; one part, the germary, being an organ for the production by cell multiplication of the germ-cells or eggs proper, the other, the vitellarium, much more conspicuous and usually consisting of a definite number of large cells, producing yolk material for the growth of the egg. The whole ovary is unilateral and unpaired in most rotifers; symmetrical in Asplanchnaceae, Philodinaceae and Seisonaceae. In Asplanchnaceae the germary is median, continuous at the distal end with the middle of the transverse horseshoe-shaped vitellary. In Bdelloidaceae and Seisonaceae the whole organ is paired, the germary proximal, the vitellary next the cloaca. As a rule, the wall of the ovary is continued into a uterine tube opening into the cloaca; but in Philodinaceae this is absent, and the young are free in the body cavity and escape by perforating the cloacal walls. The male organs are usually a testis, a large seminal bladder and a protrusible penis. The males are unlike the females in most species; only in Eosphora digitata, Rhinops vitrea, Proales werneckii, and the Seisonaceae a complete digestive system is present. Frequently the foot is ciliated at the tip, as in the young of tubicolous forms.

The males of rotifers are of relatively rare occurrence, except in the genus Asplanchna, where they were first recognized as such by Brightwell in 1841; though those of Hydatina had long since been seen and described as a distinct genus. Despite their rare occurrence, the males of over one hundred and twenty species have now been recognized, and we may well believe that all species will be found to present males. This statement may seem to need qualification; for the male of no Bdelloid has been seen, and there is but a doubtful record of “winter-eggs” in this group. But possibly, as in Seisonaceae, the males resemble the females, and have escaped recognition. It may, however, well be that the capacity for wintering in the dry state has physiologically replaced the need for resistant fertilized eggs. Insemination takes place either by the introduction of the penis into the cloaca of the female, or by the puncture of the bodywall of the female by the penis, and the injection of the sperm into

the body cavity, whence the single spermatozoa must make their way to the eggs. The females habitually produce eggs without impregnation, which again habitually develop into females, more rarely into males. These unfertilized eggs develop directly, often in the uterus. In other cases the eggs are liberated earlier and adhere to the foot, or are hatched within the tube (fig. 3, b, c). The impregnated eggs undergo a very partial development in the mother, and these pass into a state of rest, for which they are furnished with a dense shell. They always give rise to parthenogenetic females (see ). The thin-walled eggs are often termed “summer-eggs,” the fertilized ones “winter” or “ephippial” eggs (by parity with the phyllopod, q.v.). But the appearance of males seems to be as much associated with those of summer drought as of winter cold. No adequate knowledge of the conditions under which males arise has been established. The phenomenon of seasonal dimorphism is of especial moment for the plankton dwellers. Not only is the appearance of males regular, but the forms of the females at different times of the year may be so distinct as to have led them to be classed as distinct species.

Development.—The egg is holoblastic, but the segmentation is very unequal, recalling that of marine annelids and of molluscs. Gastrulation takes place by epiboly, and the stomodaeum (oral invagination—mastax pharynx) takes place in two stages of the region of the closed blastopore. Unlike the molluscs and annelids, however, the cloacal invagination lies outside this region, and the foot is formed by an elongation of the end of the body between the two apertures. The nerve ganglion is formed by an ingrowth of epiblast, and so are the pedal glands. The body cavity is the primitive blastocoele.

Relationships and Morphology.—Passing over the earlier authors who regarded this group as allied to Infusoria, a view first contested by Dujardin, T. H. Huxley viewed them as equivalent to and on a level with the larvae of Echinoderms, and of such other trochophore larvae as resembled these, a view generally adopted. But it became more and more apparent that the larvae of this category developed mouth, gut and anus by the closure in the middle of such a slit-like blastopore opening into a sack-like stomach as is seen in the larvae of Turbellaria and Nemertina. The extra-blastoporic opening of the cloaca leads us to a very different view, which finds negative support in the failure of previous morphologists to adapt the details of development and of the structure of the disk to their identification of “trochus” and “cingulum” with the preoral and postoral wreaths of the trochophore larva. We homologize the rotifer with the Turbellarian larva (fig. 8, A), and with the preoral or upper part of the trochopore (fig. 8, E, F). Its adhesive foot is paralleled by a cup-shaped ciliated depression, possibly nervous, found in all the larvae cited, except some Echinoderms, and which in Asterids and Crinoids actually serves as an organ of attachment. This view obviates the deed for assuming the complicated flexures of the wreath which has to be done on other assumptions (see, Encycl. Brit. ed. 9). Thus Trochosphaera (fig. 8, D) (which has a male of the same type as Melicerta, &c.) is an extremely modified type, and its resemblance to the trochophore larva of Lepadorhynchus or Polygordius is only superficial. We may note that it was long since shown that the apical organ (at first assumed to be the brain) of these larvae was innervated from an anterior thickening of the circular nerve ring, corresponding with the brain of Rotifers; the nerve cells immediately below the pit are the ordinary bipolar