328 THE ECHINOIDEA 



locomotion, the apical system of plates becomes compact, elongate, 

 disjunct, or rudimentary, and a bilateral symmetry replaces the 

 primitive, pentameral symmetry. At first the jaws are retained ; 

 but as the body becomes bilateral, the mouth is constricted, and 

 room for play of the jaws is lessened. No doubt all Echinoids get 

 a proportion of nourishment from the mud and sand which they 

 swallow ; but as their jaws become smaller, and they can browse 

 less effectively, the importance of this food-supply becomes more 

 important. The development of a projecting under lip below the 

 mouth was an advantage to the Echinoid, by enabling it to swallow 

 more food. Hence the Irregular Echinoids began with teeth and a 

 central mouth a type first met with in the lower Jurassic ; later 

 on, in the Jurassic, came the second type, in which there are no 

 jaws and the mouth is eccentric ; the former is the order Gnatho- 

 stomata, and the latter the order Atelostomata. The Gnathostomata 

 began in the Jurassic with the genus Pygaster, which differs from 

 the regular ectobranchiate Echinoids only by the anus opening 

 behind the apical system ; the Pygastridae were succeeded in the 

 Cretaceous by the Discoidiidae, from one genus of which, Proto- 

 cyamus, there is an easy passage to the Fibulariidae, and thus to 

 the sub-order Clypeastrina. 



The order Atelostomata has apparently also been derived from 

 a genus allied to Pygaster. The jaws are lost, and the apical 

 system either remains compact or becomes elongate : the former 

 series possibly began with Galeropygus, whence the rest of the 

 asternate forms were derived. The series with elongate apical 

 systems began with some such genus as Hybodypeus, which led the 

 way to the Collyritidae and Echinocorythidae, whence the higher 

 Spatangid Echinoids descended. 



LITERATURE OF ECHINOIDEA. 



1. Agassiz, A. 1872-74. (Revision of the Echini.) 111. Cat. Mas. Comp. Zool. - 



Harvard, No. vii. 2 vols. 762 pp. 94 pis. 



2. 1881. (Report on the Echinoidea dredged by H.M.S. Challenger 



during the years 1873-76.) Rep. Chall. Exped. Zool., vol. iii. 321 pp. 

 45 pis. 



3. 1883. (Report on the Echini. Results of dredging by the Blake, 



No. xxiv. pt. 1.) Mem. Mus. Comp. Zool. Harvard, vol. x. No. i. 94 

 pp. 32 pis. 



4. Agassiz, L. 1835. (Prodrome d'une Monographic des Radiaires ou Echino- 



dermes.) Mem. Soc. Sci. Nat. Neuchatel, vol. i. pp. 168-199. 



5. Agassiz, L., Desor, E., and Valentin, G. 1838-42. Monographies d'Echino- 



dermes vivans et fossiles, Nos. i.-iv. ; and Anatomic, No. i. 4to, Neuchatel. 



6. Agassiz, L., and Desor, E. 1846-48. (Catalogue raisonne des families, des 



genres et des especes de la classe des Echinodermes. ) Ann. Sci. Nat. 

 Zool., ser. 3, vols. vi., vii., and viii. 167 pp. 2 pis. 



