27-4 EMBRYOLOGY 



at first is rather evenly ciliated, one having a preoral and 

 a perianal band of cilia, which is substantially in the 

 Trochophore stage. 



The young larvse of Terebella Meckelii are at first spherical, then 

 elongate a little, and become covered with an even coat of cilia, which 

 leaves bare only the small part of the anterior end of the body lying in 

 front of the eyes. Later they become pyriform, and the cilia now cover 

 only the voluminous anterior part of the body, whereas the posterior 

 region is destitute of them. It is only in a later stage of development 

 that a perianal row of cilia makes its appearance. In this stage it re- 

 sembles the previously described larva of Lumhriconereis, The ciliation 

 is gradually confined to a preoral band, a perianal band, and a ventral 

 ciliated groove (Salensky). To be sure, the outer form of the larva is in 

 this case, on account of the small size of its bell, not that of a typical 

 Trochophore ; nevertheless nothing prevents us from comparing it to one 

 that has already begun its metamorphosis into the worm. In front of the 

 mouth lies the preoral band of cilia ; the intestine has the usual shape ; 

 at the posterior end, in the vicinity of the anus, is found the perianal band 

 of cilia. About midway between the anterior and posterior ciliated bands 

 appear indications of the two first segments, behind which others soon 

 follow. They become noticeable externally by the development of pro- 

 tuberances, which are studded with setae. The worm grows in length ; 

 evaginations at its anterior end form the tentacles ; it secretes the tube 

 and attaches itself. 



The larvae of the Chcetopteridce, known as Mesotrochce, also 

 arise from uniformly ciliated embryos. In Chcetopterus 

 pergamentaceus, which at first is even- 

 ly ciliated, there is formed a tuft of 

 cilia at the anterior end of the larva, 

 and gradually a ring also of cilia, en- 

 circling the body at about the middle 

 (Fig. 123). The inside of the larva is 

 pretty well filled by the large sac-like 

 intestinal canal. The larva of Telep- 

 FiG.i23.-So-caiiedme- «^^'^« costarum is similar, only that it 

 sotrochai larva of Chmtop- lacks the anterior tuft of cilia. On 



terns pergamentaceusi. iter ^^^ ^^^^^ ^^^^ ^^^ ^^^^^ ^f p;^^;^. 

 Wilson), m, mouth. _ ' _ _ ^ '' 



chcetopterus socialis also exhibits stouter 

 cilia at the anterior end ; it possesses two ciliated bands 

 which lie close to the posterior end. A preoral band of cilia 

 is not present in these larvae, and the middle one cannot be 



