Introduction to Ajiimal Morphology. 



125 



most in Holothurise. The fibres are usually unstriped, 

 except in the jaw-muscles of Echinus, and the longi- 

 tudinal bands in Synapta. A non-ciliated, pseudhae- 

 mal circulatory system exists in most species, consist- 

 ing of a ring-like (arterial ?) canal, between the nen'ous 

 and water-vascular rings around the oesophagus, 

 with sometimes a second circum-anal (venous r) circle, 

 and a fusiform muscular heart, uniting the two 

 circles. 



The sexes are separate (except in Synapta^ 

 Molpadia, &c.) The eggs are small, consisting of a 

 shell, albumen, and a fine-grained yelk ; development 

 begins by the disappearance of the germinal vesicle, 

 and miay be direct, but usually the Q^<g produces a 

 larva (pseudembryo, W. Thomson), of which (except 

 in Crinoids and Holothurians) only the part around 

 the digestive canal develops into the mature form, 

 the rest being only provisional. This larva is 

 bilaterally symmetrical, and resembles that of some 

 of the Annulosa. In this 

 stage all Echinoderms are 

 somewhat like in struc- 

 ture. From the egg is 

 emitted a ciliated planula, 

 whose cilia become re- 

 stricted to transverse or 

 lateral bands, which often 

 elongate into processes. 

 A stomach and intestine 

 form within, beginning as a 

 pouch, convex backwards, 

 opening at first by an 



, . - . Plutcus larva of Ophiolepis ; s, skeleton ; 



anus, but soon developmg a ^, pedicein ; .', stomach ; m, mouth. 



Fig. 19. 



