416 



EMBRYOLOGY 



plete covering of cilia of the first few days, it subsequently 

 acquires five ciliated rings, which encircle the body trans- 

 versely, and a tuft of long cilia at the anterior end. The 

 most anterior of the ciliated rings is less sharply marked 

 than the others (Bury), owing to which earlier authors 

 spoke of only four rings of cilia. The larva swims with the 

 tuft of cilia directed foi'wards. [Under the tuft lies a 

 thickening of the ectoderm which was recognized already by 

 Nachtrieb (No. XXa, Appendix to Literature) as an apical 



plate. It has been confirmed 

 as such by the recent thorough 

 investigations of Seeligbr (No. 

 XXVI.). Indications of an 

 apical plate also appear in the 

 younger stages of other Echi- 

 noderm larvee (Asteroidea and 

 Echinoidea). — K.] If we com- 

 pai-e the larva of Antedon with 

 that of the other Echinoderms, 

 in which the blastopore be- 

 comes the anus, we must look 

 upon the end opposite the anus 

 — namely, the one provided 

 with the tuft of cilia — as the 

 anterior end of the larva, and 

 more especially because the so- 

 called larval mouth lies nearer 

 to this end. Recent authors 

 designate as the mouth a cili- 

 ated depression which lies on 

 the ventral side between the 

 second and third rings of cilia 

 (Fig. 197). This pit does not 

 represent an actual mouth-opening, for it is not connected 

 with the intestine, but the so-called vestibule is subsequently 

 foi^med at this point, and the mouth-opening arises at its 

 bottom. 



A small pit is found on the ventral surface near the 

 anterior end of the larva, which later serves the larva in 



l,nL 



Fig. 197.— La,rva of Antedon rosacea, 

 with ciliated rings and tuft of cilia 

 (after GoKiTE and W. Thomson). The 

 earliest skeletal pieces are already 

 formed as fenestrated plates. Gi-, pit 

 which serves the larva in attaching 

 itself ; Lm, the so-called larval mouth. 



