288 PHYLUM ECHINODERMATA. 



organs. It is not clear whether there is any radial branch of 

 this system into the arms. The intestinal network and that 

 over the axial organ communicates with the circumoral tract, 

 and on the circumoral tract on the rectal side is a special 

 development of this tissue containing cellular elements and 

 called the spongy organ. 



The chambered organ is a portion of the embryonic coelom 

 (p. 156). In the adult it is completely cut off from the rest of 

 the coelom and consists of five radially disposed chambers 

 separated by interradial septa, the whole being surrounded by 

 the central organ of the apical nervous system (p. 283). It is 

 placed at the apex of the calyx, in the Comatulids in the 

 centro-dorsal plate. The chambered organ is continued, with 

 its nerve investment, into the stalk in the stalked forms, and into 

 the cirri if such are present. 



The sacculi are globular sacs containing highly refractile 

 spherules. They are found in the connective tissue at the edge 

 of the ambulacral grooves of the arms, pinnules and calyx, and 

 sometimes in other parts. Each spherule is in its origin related 

 to one cell, the remains of which can be traced round it. Their 

 meaning and function is unknown. They are absent in Actino- 

 metra, Thaumatocrinus and Holopus. 



The generative organs may be described under two heads : 

 {!) the generative rachis, and (2) the gonads. These two struc- 

 tures are continuous and form part of one structure of which the 

 rachis is the sterile portion. The rachis is contained in the 

 arms (Fig. 197) : it is a cord of cells containing a small lumen 

 and surrounded by vascular tissue. It lies in the genital divi- 

 sion of the arm body-cavities and is continuous in the disc, 

 through a circular cord (Antedon) or a network of strands 

 (Pentacrinus), with the axial organ. The generative organs are 

 developments of the terminal portions of the generative rachis, 

 i.e. of the portion contained in the pinnules. In exceptional 

 cases the generative rachis of the arms also gives rise to 

 generative cells. In the pinnules the generative rachis swells 

 up, its cavity becomes larger, and its lining cells become 

 ova or spermatozoa (Fig. 198). These escape into the water 

 probably by dehiscence. 



So far as is known Crinoids are always of separate sexes, and 

 the development, which has been followed in Antedon only 



