584 THE ANIMAL KINGDOM. 



The body of the adult is formed by a lengthening and segmentation of the 

 region between the post-oral ciliated ring and anus. The Trochosphere is 

 probably derived from an organism (Trockosooti) resembling the early stage 

 of a Turbellarian larva (Miiller's larva), from which the Nemertean Pilidium 

 is also undoubtedly derived '. The prostomium of the Trochosphere must 

 be taken to correspond with the aboral apex of the primarily ovate body 

 of the Turbellarian larva, which undergoes a tilt in one direction, while the 

 mouth, which is at first situate at the opposite end of the same vertical axis, 

 comes to lie behind the apex ; at the same time that side of the originally 

 symmetrical body which is now opposed to the apex in its altered position 

 commences to lengthen and grow into the body of the worm. The 

 characteristic prae-oral ring of the Trochosphere is derived from the 

 equatorial band of the simple larva. The body of the Vermian, like the 

 Molluscan Trochosphere, grows in such a way that its posterior apex lies 

 on the oral aspect of the equatorial ring, and the anus, which is never pre- 

 sent in the Turbellarian is consequently, when formed, below, i.e. on the 

 oral side of the ring. On the contrary, in the Enteropneustan Tornaria 

 and the larval form of all Echinoderms (except Crinoideat\ it is on the 

 apical or aboral side of the equatorial ring, which in these instances takes a 

 longitudinal direction. 



The development of the Nemertean from the Pilidium, or from the 

 larva of Desor^ and of the Hirudinea, is remarkable for the new formation 

 of the permanent ectoderm and the discarding of the larval ectoderm, pro- 

 visional nervous system, and musculature. The Hirudinea, like the Oligo- 

 chaeta Chaetopoda, have no special larval form, a want probably due to an 

 abbreviation of development. The Tornaria of some species viBalanoglossus 

 has a resemblance to the Bipinnaria of Aster oidea. It differs from it as from 

 the Holothurian and Echinoid larvae, and agrees with the Trochosphere in 

 the possession of optic organs and a ganglionic rudiment at the apex of the 



first-named. See Hatschek on the head of Polygordius and the Trochosphere of Eupomatus, in 

 Arb. Zool. Inst. Wien, vi. (i), 1885. Attention was first drawn to the prae-oral circular nerve by 

 Kleinenberg. Its resemblance to the nerve following the edges of the ciliated lateral lobes in the 

 PiUcHum, as described by Salensky, is very striking, especially from the point of view of the possible 

 origin of the Trochosphere from some Pilidium-V&it, ancestor. See Salensky, Z. W. Z. xliii. 1886. 

 It is generally held that the ventral nerve-cord originates independently of the supra-oesophageal 

 ganglion. Hatschek, whose observations on Polygordius and Echiurus are followed in the text, 

 doubts this fact. At any rate such an origin is not primitive. The division of the digestive tract 

 into stomodaeum, archenteron and proctodaeum, e. g. in Eupomatus, is probably typical. In the 

 larval Serpula, however, as described by Conn (Z. A. vii. 1884), the blastopore lengthens out 

 aHtero-posteriorly, closes, and at its two opposite ends appear the mouth and arms ; the middle 

 region corresponding to the median ventral line joining the two. It is possible, however, that 

 an epiblastic stomo- and procto-daeum may be formed subsequently. Compare Hatschek 's account 

 of the formation of the stomodaeum in Eupomatus (loc. cit. supra). The Trochosphere of the last- 

 named possesses a pair of otocysts, and, like all Serpulid larvae, an adanal vesicle formed by a 

 vacuolated ectoderm cell according to Hatschek, by an endoderm cell according to Conn. 



1 The Chaetopod larva (of a Clymenid ?), known as Mitraria, has a remarkable resemblance to 

 a Pilidium. See Metschnikoff, Z. W. Z. xxi. 1871, p. 233. 



