ENDOCYCLICA 537 



descri])ed by the Sarasins/ who regarded its structure as a ])roof 

 that Ecliinoidea were derived from Holothuroidea. Both palae- 

 ontology and embryology have, however, yielded strong evidence 

 that Echinoidea were derived from Afteroidea, and hence there 

 is ground for believing that Holothuroidea are descended from 

 primitive Echinoidea and not Tice versa. The Echinothuriidae 

 may perhaps be regarded as showing the first steps in the 

 change, and thougli possibly not closely related to the actual 



peristome. 



mouth 



Fifi. 239. — View of peristome of Asthenosoma liydrix. amh, Ainbulacral plates on the 

 lower edge of the corona ; inter, lower plates of the interanibulacral area. (From 

 Wyville Thomson.) 



ancestors of the Plolothuroidea, they at any rate show parallel 

 modifications. 



Fam. 3. Saleniidae. — Endocyclica with a large peristome 

 and periproct. The peristome is covered with thin, scattered, 

 irregular plates. There are five pairs of special buccal tube-feet, 

 ^ach supported by a special plate, and there are external gills. 

 TIh! periproct is excavated in the side of a large central pentagonal 

 plate. 1 1 is covered with fifteen or twenty plates.^ The ambulacral 

 plates are separate as in the Cidaridae, but occasionally adhere in 

 pairs near the peristome. The interanibulacral plates also, as in 

 Cidaridae, each bear one large primary spine surrounded by a 



' Reference on p. 528 n. 



- Tills account of the periin-oct is different from that ordinarily given. It is 

 based on the most recent examination of this family — Agassiz, " I'anamic Deep- 

 sea Echini," Mem. Mus. Cump. Zwl. xxxi. 190i, p. 36. 



