150 CLASS iv. 



Family III. Echinidea. Body sub-globose or depressed with- 

 out radiant lobes. Mouth and anus distinct. Mouth inferior. Inte- 

 gument calcareous, beset with moveable spines. 



Sea- Urchins. Compare on this family (besides the Monographies 

 tfEchinodermes of AGASSIZ cited above) JAC. THEOD. KLEIN Natu- 

 ralis dispositio Echinodermatum cum tab. Gedani 1734, 4to. Ordre 

 naturel des Oursins de Her et fossiles par M. THEODORE KLEIN, 

 Paris 1754, 8vo. av. fig. (Many of KLEIN'S figures are copied in the 

 Encyclopedic methodique, Vers.) M. VAN PHELSUM, Brief an C. 

 NOZEMAN over de gewelv-slekken ov Zee-egelen. Met 3 pi., Hotter- 

 dam, 1774, 8vo. CH. DESMOULINS, Etudes sur les Echinides, 

 Bordeaux, 1835 1837, 8vo. 



The shell of these animals consists of an arrangement of plates 

 having a pent- or hexangular form. They compose ten girdles, each 

 made up of two rows of such plates. Five of the girdles, commonly 

 narrower than the others, have two rows of small apertures and 

 alternate with these. The rows of apertures are named ambulacra : 

 they either entirely surround the periphery (ambulacra perfecta), or 

 are found only on the uppermost part, resembling in their arrange- 

 ment a star or five-petalled flower (ambulacra circumscripta). By 

 these apertures the tentacles or ambulacral tubes are exserted, 

 of which we have treated above. The Sea-Urchins effect their 

 movements by means of these tubes 1 , they appear to have a 

 great power of elongation, to be able to stretch farther than the 

 extremities of the rigid spines, which in certain species are some 

 inches in length. Around the anus are five larger apertures (in 

 some genera only four) which are the outlets of the oviducts or 

 efferent vessels ; they are situated in as many pentagonal calcareous 

 plates, with the point directed outwards, of which one, larger than 

 the rest and of a different structure, corresponds to the calcareous 

 plate (madrepore-plate) of the Sea-stars, as BASTER had previously 



of Natural History vi. 1841, pp. 175 184, pp. 275 290. Want of space prevents 

 our noticing the numerous generic names of GRAY ; some genera agree with those 

 of MUELLER and TROSCHELL, of which a more detailed notice by AGASSIZ may 

 be found in the preface to the second number of his Monoyraphies d'Echinodermes, 

 pp. 5, 6. 



1 GANDOLPHE Quelles sont lesjambes des Oursins? Mem. de VAcad. des Sc. de Paris 

 pour 1709, Histoire, p. 33. With his observations those of BASTER, TIEDEMANN and 

 others completely agree ; AGASSIZ, who at one time considered the spines to be organs 

 of motion, and doubted that such was the office of the ambulacral tubes, has since 

 renounced that opinion. 



