98 



INVERTEBRATE ANIMALS. 



sides of the body. In all Echinoderms there is a water-vascular 

 system of tubes, which is termed the " ambulacral system," which 

 generally communicates with the exterior, and which in most cases 

 is used in locomotion. An alimentary canal is always present, and is 

 always comjjletely shut off from the general cavity of the body. There 

 are always distinct organs of reproduction, which are almost always 

 placed in different individuals, so that the sexes are distinct. The nei- 

 vous system is in the form of a ring surrounding the gullet and send- 

 ing branches in a radiating manner to different parts of the body. 



One of the most characteristic points about the Echinoderms is 

 their mode of development. In most cases, namely, the embryo is 

 ii little oval body covered all over with cilia ; but in the course of 

 growth the cilia become restricted to transverse bands or to definite 

 outgrowths of the body. The embryo is now often somewhat coni- 

 cal or pyramidal in shape, with long projecting processes, and may 

 be supported by a provisional internal skeleton (fig. 60). An ali- 



Fig. 60. — Larva of Edtinns. (After J. ^^hiller.) A, A, Front arms with their internal 

 slieletou; F, F, Arms of the month - process ; B, Pnsterior side-arms: a Month; 

 ".' CEsopliaRUS ; h Stomach ; &' Intestine; d Ciliated bands ; // Ciliated epaulets ; 

 c Disc of the futnre Echinus. 



mentary canal with a distinct mouth and anvis is present, and the 

 embryo is thus divided into two bilatei-ally symmetrical halves. 

 This singular embryo was originally described, in the case of the 

 Sea-urchins, as an independent animal, under the name of " Phtteus." 

 The special peculiarity of the further development of the embryo is 

 that the adult animal is ultimately developed out of a mass of active, 

 growing protoplasm which appears on one side of the stomach of 



