METAZOA ECHINODERMA. 191 



consisting of two ambulacra (figs. 2, 4), and the ambu- 

 lacral feet with sucking discs are restricted to this side 

 and extend in pairs towards the posterior end of the 

 body (figs, i, 2, 4). On the dorsal side there are tubu- 

 lar organs which, as they perform a different function 

 from the ambulacral feet, are without sucking discs (fig. 

 3 ; in fig. i these organs are broken off, but the four 

 openings on the anterior part of the body show their 

 position). 



In many Elasipoda the water-vascular system com- 

 municates with the exterior by means of the madreporic 

 body. Moreover, the circular ring around the mouth is 

 made up of simple spicules which are separated from one 

 another. 



The internal respiratory organs lack the usual tree-like 

 form, and the tentacles are few in number, usually not 

 more than ten in the Elpidiidae, the most generalized 

 family. 



While the Elasipoda have retained the larval characters 

 more than the other members of the class, still we cannot 

 fail to see what Theel l has already pointed out, that these 

 Echinoderms are more like the specialized forms of most 

 invertebrates in several important particulars. They are 

 bilaterally symmetrical. They have a distinct antero- 

 posterior axis and a ventral side differentiated from the 

 dorsal. They have a small number of locomotive organs 

 and these have a definite position. 



The remaining families of Holothuroids the Pedata 

 and Apoda when young resemble the Elasipoda. 

 Most of these forms belong to the shallow waters and 

 they have become greatly modified to meet widely dif- 

 ferent conditions. 



At first the madreporic body opens on the exterior, but 

 later the connection is lost and the canal ends blindly in 

 the interior. 



iChall. Rep., Zool., IV, part 13, 1882, p. 147. 



