ix PHYLUM ECHINODERMATA 377 



Algae ; but the regularity of their arrangement is opposed to 

 such a view. It has been suggested with more appearance of 

 probability that they may be masses of reserve materials, stored 

 up for the nutrition of the animal. 



The reproductive organs ovaries or testes, as the case may 

 be are lodged in the dilated bases of the pinnules, which become 

 considerably enlarged as the ova or sperms mature, those next to 

 the bases of the arms alone remaining sterile. When mature, the 

 sexual elements escape by means of short ducts. Each gonad 

 is one of the terminal parts of a system of tubes lined by an 

 epithelium, and extending from a central part or genital stolon 

 {gen. si) lodged in the vascular plexus that surrounds the oesopha- 

 gus and connected dorsally with the chambered organ, outwards 

 through the arms, the terminal portions, lying in the pinnules, 

 becoming dilated to form the reproductive organs, and the cells 

 of their epithelium becoming developed into ova or sperms, while 

 the rest constitute a non-fertile connecting rachis. This system 

 is enclosed throughout by a plexus of blood-vessels. 



Like the rest of the Echinoderms, the Feather-star undergoes a 

 metamorphosis (Fig. 316). The larva is at first oval and 

 covered uniformly with cilia. Afterwards the cilia become 

 restricted to four transverse bands with a bunch of longer cilia 

 .at one end ; the body becomes bent towards the ventral side, 

 ossicles begin to be formed, and the posterior extremity becomes 

 drawn out into a narrow process in which supporting ossicles 

 soon appear. At the end of this posterior, or more correctly 

 dorsal, process, a terminal disc is formed, and by means of this 

 the larva fixes itself, the process forming a supporting stalk. 

 The larva now assumes the form to which the term " pentacrinoid" 

 has been applied, owing to the fact that, in its most essential 

 features, it resembles the adult form of Pentacrinus, one of the 

 stalked Crinoids (See Fig. 314), with a central disc, giving off five 

 bifurcated arms with their pinnules, and supported on a narrow 

 stalk springing from the middle of the dorsal surface. This fixed 

 pentacrinoid larva passes into the adult, free-swimming Feather- 

 star by the development of the dorsal cirri, the greater elongation 

 -of the arms, and the absorption of the stalk, the uppermost ossicle 

 of which is represented in the adult by the centro-dorsal ossicle. 



5. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Echinodermata are radially symmetrical animals, the radial 

 arrangement of whose parts imperfectly conceals a more obscure 

 and more primitive bilateral symmetry. The surface is covered 

 with an exoskeleton of calcareous plates or ossicles, which 

 usually support a system of movable or immovable calcareous 

 spines. There is a large body-cavity or coelome, and well-developed 



