Introduction to ^iiiiifiat J^Iorphology. igi 



vesicles of Echinoderms. This portion has been described 

 as a special tentacular or aquiferous system. Similar caeca 

 may be appended to the dorsal vessel ; but when there are no 

 tentacles, this canal breaks up into fine branches anteriorly. 

 In those with no vascular system (except Halicryptus) there 

 is a posterior opening, capable of voluntary closure, into the 

 body cavity, whereby water can pass in and out for aerating 

 purposes (?) A similar anterior opening I have found co- 

 existing with a vascular system in several genera. The fluid 

 in the circulatory system may be colourless, pale red, blue or 

 violet. There may be a similar fluid in the body cavity con- 

 taining amoeboid, flagellate, and ciliated (?) corpuscles. 



Priapulus has at its hinder end a fimbriated tuft, which 

 may be respiratory. Echiurus has opening into the intestine, 

 at its lowest part, branched organs, receiving vessels from the 

 ventral trunk, like the tree-like organs of Holothurians, which 

 are either excretory or respiratory. There is a rudiment of 

 this in Sipunculus and others, and in Bonellia it has many 

 openings into the cceloma. The tentacles of Sipunculus also 

 act as breathing organs. There are certain ventral ciliated 

 pouches (often four) homologous with the segmental organs of 

 Annelids. In Echiurus all the four may be utilised as ex- 

 cretory ducts for the sex-organs. In Sternaspis, one pair 

 only is connected therewith, the others containing a yellow 

 material (urinary ?) Thalassema has only one pair existing 

 corresponding to the former pair in Sternaspis ; these open 

 inwards by fine spiral canals. In Sipunculus there is one 

 pair representing the latter pair in Sternaspis, and non-sexual. 

 In Bonellia there is only one, forming a funnel-shaped 

 uterus, and representing the right (?) of the sexual pair of 

 Sternaspis. 



The males are fewer than the females, often 

 dimorphic (in Bonellia as a planaria-like form, living 

 in the female sexual organs, Kocvalewsky). The sexual 

 organs are string-like, simple or in pairs (or a ciliated 

 furrow on the intestine Sipunculus, Peters), dehiscing 

 into the body cavity. A few are hermaphrodite 



