Invertebrate Gallery of the Indian Museum. 107 



The principal internal characters of the Clypeastroida 

 are exposed in the spirit-dissection of Echinodiscus Ixvis 

 (Case 33), where the mouth is seen to be armed with teeth 

 which, however, are more horizontal than vertical ; the 

 " pyramids " are broad and flat, and are united together to 

 form a large pentagonal plate, and there are no braces 

 and compasses. The nature of the mouth-parts, as of the 

 " perignathic girdle " and of other parts of the interior of 

 the test, is shown in several dry preparations in Case 32. 



In the Spatangoida the test is very commonly heart- 

 shaped. Looking at such a form as Lovenia^ Cases 33-34, 

 on the under (ventral) surface, at the broader end of the 

 " heart," we find the toothless valvular opening of the 

 mouth, while at the apex of the "heart'' the periproct is 

 seen. On the upper (dorsal) surface, near the centre, we 

 notice the five small basal (genital) plates, three or four of 

 which are perforated, while the fifth, which is imperforate, 

 forms a small " madreporite ". Radiating from the genital 

 plates is an irregular ambulacral " rosette "of five rays, four 

 of which are conspicuous by their breadth and by the size 

 of their pores, while the fifth by reason of the minuteness of 

 its pores is at first sight likely to be overlooked. Owing 

 to the absence of teeth no "jaws" and no "perignathic 

 girdle " are found among the Spatangoida. 



The Echinoidea are found in all seas, and at all depths 

 from tide-marks to the abysses of the ocean. In the Indian 

 region the Regular Echinoids are met with in profusion on 

 the coral reefs — Cidaris dind. Phor?nosoma, however, being 

 most abundant at depths of 150 to 500 fathoms: the Clype- 

 astroida are very common in shallow muddy water, as for 

 instance along the estuarine coasts of the great rivers of the 

 Bay of Bengal : while the Spatangoida have been obtained 

 in greatest number on muddy bottoms in 400 to 600 

 fathoms. 



Such being their range, their habits of life are various : 

 the shallow- water forms live on Molluscs and Crustacea, on 

 sea-weed and even on decaying vegetable matter ; the deep- 



