Page:EB1911 - Volume 16.djvu/246

 the apical plate (1). This usually carries a tuft of long cilia or sensory hairs, and sometimes rudimentary visual organs. Mesoblastic bands are present, proceeding a short distance forwards from the anus on each side of the middle ventral line (6), and at the anterior end of each of these structures is a tube (5) which more or less branches internally and opens on the ventral surface. The branches of this tube end internally in peculiar cells containing a flame-shaped flagellum and floating in the so-called body cavity, into which, however, they do not open. These are the primitive kidneys. The body cavity, which is a space between the ectoderm and alimentary canal, is not lined by mesoderm and is traversed by a few muscular fibres. Such a larva is found, almost as described, in many Chaetopods (fig. 1), in Echiurus (fig. 2), in many Gastropods (fig. 3), and Lamellibranchiates (fig. 4). This typical structure of the larva is often departed from, and the molluscan trochosphere can be distinguished from the annelidan by the possession of a rudiment at least of the shell-gland and foot (figs. 3 and 4); but in all cases in which the young leaves the egg at an early stage of development it has a form which can be referred without much difficulty to the trochosphere type just described. A larva similar to the trochosphere in some features, particularly in possessing a preoral ring of cilia and an apical plate, is found in the Polyzoa, and in adult Rotifera, which latter, in their ciliary ring and excretory organs, present some resemblance to the trochosphere, and are sometimes described as permanent adult trochospheres. But in these phases the resemblance to the typical forms is not nearly so close as it is in the case of the larva of Annelida and Mollusca.

In the Echinodermata there are two distinct larval forms which cannot be brought into relation with one another. The one of these is found in the Asteroids, Ophiuroids, Echinoids and Holothuroids; the other in the Crinoids.

The first is, in its most primitive form, a small transparent creature, with a mouth and anus and a postoral longitudinal ciliated band (fig. 5, A). In Asteroids the band of cilia becomes divided in such a way as to give rise to two bands, the one preoral, encircling the preoral lobe, and the other remaining postoral (fig. 5, B). In the other groups the band remains single and longitudinal. In all cases the edges of the body carrying the ciliary bands become sinuous (fig. 6) and sometimes prolonged into arms (figs. 7-9), and each of the four groups has its own type of larva. In Asteroids, in which the band divides, the larva is known as the bipinnaria (fig. 7); in Holothurians it is called the auricularia (fig. 6); in Echinoids and Ophiuroids, in which the arms are well marked, it is known as the pluteus, the echinopluteus (fig. 9) and ophiopluteus (fig. 8) respectively.

All these forms were obviously distinct but as obviously modifications of a common type and related to one another. They present certain remarkable structural features which differentiate them from other larval types except the tornaria larvae of the Enteropneusta. They possess an alimentary canal with a mouth and anus as does the trochosphere, but they differ altogether from that larva in having a diverticulum of the alimentary canal which gives rise to the coelom and to a considerable part of the mesoblast. Further, they are without an apical plate with its tuft of sensory hairs.

In Crinoids the type is different (fig. 10), and might belong to a different phylum. The body is opaque, and encircled by five ciliary bands, and is without either mouth, anus or arms, and there is a tuft of cilia on the preoral lobe. A resemblance to the other Echinoderm larvae is found in the fact that coelomic diverticula of the enteron are present.

The larvae of two other groups present certain resemblances to the typical Echinoderm larvae. The one of these is the tornaria larva of the Enteropneusta (fig. 11), which recalls Echinoderms in the possession of two ciliary bands, the one preoral and the other postoral and partly longitudinal, and in the presence of gut diverticula which give rise to the coelom; but, like the trochosphere, it possesses an apical plate with sensory organs on the preoral lobe. The resemblance of the tornaria to the bipinnaria is so close that, taking into consideration certain additional resemblances in the arrangement