150 CLASS IV. 
Family III. Echinidea. Body sub-globose or depressed with- 
out radiant lobes. Mouth and anus distinct. Mouth inferior. Inte- 
eument calcareous, beset with moveable spines. 
Sea-Urchins. Compare on this family (besides the Wonographies 
d’ Echinodermes of AGasstz cited above) Jac. THEop. Kier Vatu- 
ralis dispositio Echinodermatum cum tab. Gedani 1734, 4to. Ordre 
naturel des Oursins de Mer et fossiles par M. Tueopore Kien, 
Paris 1754, 8vo. av. fig. (Many of Kuer’s figures are copied in the 
Encyclopédie methodique, Vers.) M. Van Puernsum, Brief an C. 
Nozeman over de gewelv-slekken ov Zee-egelen. Met 3 pl., Rotter- 
dam, 1774, 8vo. Cu. Desmourins, tudes 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', 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 Basrer had previously 
of Natural History vi. 1841, pp. 173—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 Monographies d’Echinodermes, 
pp. 5, 6. 
1 GANDOLPHE Quelles sont les jambes des Oursins ? Mém. de 0 Acad. 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. 
