208 PHYLUM ECHINODERMATA. 



Class ECHINOIDEA * 



Spherical, oval, or discoidal Echinoderms with a shell composed 

 of calcareous plates usually closely fitting and carrying movable 

 spines. The mouth is on the under surface, and the anus either 

 within the apical system or between the apical system and the mouth. 

 The five ambulacra are indicated by rows of pores and usually 

 extend almost to the aboral pole. There are typically five inter- 

 radial gonads. 



The body of an Echinoid is typically spherical, but is often 

 heart-shaped, oval, or flattened. There are no arm-like pro- 

 longations, but there is a five-rayed symmetry in the build of 

 the body as is shown by the disposition of the water-vascular, 

 nervous and other systems of organs. The dermal skeletal 

 plates, which are pentagonal or hexagonal in shape, are con- 

 nected together so as to form, with rare exceptions (Echino- 

 thuridae, which see, and certain Palaeechinoidea, plates of 

 posterior ambulacrum of some Spatangids, peristomial plates 

 of Cidaroida), a firm immovable skeleton, the test or shell. The 

 mouth is on the lower surface of the body, generally in the 

 centre, but sometimes shifted towards what is called the 

 anterior end ; the apical system is on the upper surface and 

 the anus is either within it (Endocyclica) or outside it (Exocyclica}. 



* A. Agassiz, Revision of the Echini, Cambridge U.S.A., 1872-4. Id. 

 Report on the Echinoidea, Challenger Reports, 1881. Id. Panamic deep- 

 sea Echini, Mem. Mus. Harvard College, 31, 1904. Cotteau, Echinides, 

 Paleontologie Francaise, 7, 9, 10, Paris 1862-79. P. Martin Duncan, 

 A revision of the genera and groups of the Echinoidea. Journ. Linnean 

 Society, 23, 1891. S. Loven, Etudes sur les Echinoides, Kongl. Svenska 

 Vetenskaps-Akademiens Handlingar, 11, 1872; a translation in Ann. 

 and Mag. Nat, History (4), 10, 1872. C. F. and P. B. Sarasin, Ub. d. Anat. 

 d. Ecliinothuriden etc. Ergebnisse Nat. Forschungen Ceylon, 1, 1888. 

 Id., Die Auge etc. der Diadeniatiden, ibid. 1, 1887. J. Muller, L. Cuenot, 

 O. Hamann, Delage Op. cit. Prouho, Recherches sur lo Dorocidaris 

 papillata, etc., Arch. Zool. Exp. et gen. (2), 5, 1888, p. 213. O. Hamann, 

 Echinoidea in Bronn's Thierreich, 1901-1905. Th. Mortensen, Echinoidea 

 (Pt. l),in the Danish Ingolf -Expedition, I, vol. 4, 1903. F. Leipoldt, Das 

 angebliche Excretions-organ der Seeigel, Z.f.w.Z., 55, 1893, p. 585. 

 S. Loven, on Pourtalesia, K. Svenska Vet. Akad. Handl. 19, 1884. Id., 

 Echinologica, Bih. Svenska Vet. Akad. ForhdL, 13, 1887. Id., Echinologica, 

 Bih. Svenska Vet. Akad. Handl., 18, 1892. H. Theel, On the development of 

 Echinocyamus pusillus, Nova Acta R. Soc. Sci. Upsala, 1892. Id., Prel. 

 ace. of the devel. of Echinus miliaris, Bih. Svenska Akad. Handl. ,28, 1902. 

 E. W. MacBride, Dev. of Echinus esculentus etc., Phil. Trans., 1903, 

 p. 285. 



