ix PHYLUM ECHINODERMATA 401 



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 cords of cells enclosed in 

 narrow tubes extending from a central part or genital stolon 

 (gen. st) connected dorsally with the chambered organ outwards 

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

 are dilated to form the reproductive organs, and the cells 

 of their epithelium become developed into ova or sperms, while 

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

 is enclosed throughout by a plexus of haemal lacunae. 



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

 metamorphosis (Figs. 349 and 350). It passes through a free- 

 swimming ciliated larval stage, which is followed by a fixed 

 stalked stage known as the " pentacrinoid " larva on account of the 

 resemblance which it bears to the adult Pentacrinus, one of the 

 permanently fixed members of the same class. This fixed penta- 

 crinoid larva passes into the adult free-swimming Feather-star by 

 the development of the dorsal cirri, the elongation of the arms, 

 and the absorption of the stalk. 



5. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Echinodermata are radially symmetrical animals, the radial 

 arrangement of whose parts imperfectly conceals a more obscure 

 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 ccelome, and well-developed alimentary, nervous, and 

 vascular systems. A characteristic system of vessels, the ambu- 

 lacral system, is connected with the locomotion of the animal, as 

 well as with other functions : the organs of locomotion are in most 

 cases elastic and contractile tubular bodies, the tube-feet, which are 

 appendages of the ambulacral system. Nearly all the systems of 

 organs of the animal partake to a greater or less extent of the 

 general radial form of the body. Reproduction is entirely sexual. 

 In the course of its development from the egg the Echinoderm 

 passes through a peculiar larval stage, in which the symmetry of 

 parts is bilateral, instead of radial as in the adult animal. All the 

 Echinodermata are marine. 



The Echinodermata are classified as follows : 



SUB-PHYLUM I.-ELEUTHEROZOA. 



Echinodermata devoid of a stalk, and always freely locomotive in 

 the adult condition : with a system of radial ambulacra in the form 

 VOL. i. D D 



