ECHINODERMATA. 



FIG. 10. OVUM OF Toxo- 



PNEUSTES VARIEGATUS WITH 

 THE PSEUDOPODIA-LIKE PRO- 

 JECTIONS OF THE PROTOPLASM 

 PENETRATING THE ZONA RADI- 



developed, and therefore remains in- 

 complete. The perforation so formed, 

 becomes the micropyle of the Holo- 

 thurian egg, which was first discover- 

 ed by Joh. Miiller. The albuminous 

 membrane just described for Holo- 

 thurians is also found in Asteroids 

 (fig- 5) an< 3 Echinoids. In these 

 groups there is no proper micropyle, 

 though in Ophiothrix a nutritive pas- 

 sage perforates the membrane at the 

 attachment of the ovum before the 



11,1 i /* riM^A 1 K.\ J. II^VJ 1111, *^i>i/V 



period when the ovum becomes free ATA (zr) . (After Se lenka.) 

 (Ludwig). The formation of the zona 



radiata has been studied by Selenka. It is secreted by the 

 protoplasm of the ovum, and has a gelatinous consistency, and 

 after it is formed the peripheral layer of the protoplasm of 

 the ovum sends out through it pseudopodia-like processes to 

 absorb nutriment from without. These 

 processes are at first large and irregular, 

 but soon become finer and finer (fig. 10), 

 and acquire a regular radiating arrange- 

 ment. They are withdrawn when the ovum 

 is ripe, but they nevertheless give rise to 

 the finely radiated appearance of the mem- 

 brane, the radii being in reality delicate 

 pores. 



In the Crinoids the generative rachis 

 consists of a tube, the epithelium of which 

 is formed of the primary germinal cells. 

 (Fig. n.) While some of these cells en- 

 large and become ova, the remainder supply 

 the elements for a follicular epithelium, 

 which is established round the ova, exactly MATURE COMATULA. 



TT , , . (From Gegenbaur, after 



as m Holotnunans. Ludwig.) 



p. Tentacle, g. Lumen of genital rachis. w. Water-vascular vessel, n. Nerve 

 cord. b. Blood-vessel on nerve cord and round genital rachis. eg. Genital canal. 

 cd. Dorsal section of the body cavity, cv. Ventral section of body cavity. 



FIG. n. TRANSVERSE 



