xxvi REPRODUCTIVE ORGANS 315 



ling-vessel is a ring blood-vessel (Fig. 76, A, C. B. V\ send- 

 ing off radial blood-vessels (Rad. B. V) to the arms. An 

 inter-radial sinus or blood-space lies alongside the stone- 

 eanal, surrounding the ovoid gland (see p. 316), and is con- 

 nected below with the ring-vessel and above with a 

 pentagonal vessel or sinus, from which inter-radial branches 

 proceed to the gonads. 



The nervous system is considerably simpler than that of 

 Polygordius. It consists, in the first place, of a pentagonal 

 nerve-ring (Fig. 76, A, Nv. R] surrounding the mouth, and 

 having the character of a mere thickening of the deric 

 epithelium. From each of its angles goes off a radial nerve 

 (Rad. Nv} which passes along the arm below the ambu- 

 lacral and blood-vessels, and is also nothing more than a 

 thickening of the epidermis, some of the cells of which are 

 modified into nerve-cells and fibres. At the end of the 

 arm the radial nerve terminates in the eye-spot. In addition 

 to this superficial nervotis system there is a deep nervous 

 system, situated internally to the former, and consisting of a 

 double pentagon round the mouth, sending off double radial 

 nerves to the arms. There are also scattered nervous 

 elements in the dorsal region of the body-wall. 



Like Polygordius, the starfish is dioecious : there is no 

 external distinction between the sexes, and even the ovaries 

 and spermaries can be distinguished only by microscopical 

 examination. There are five pairs of gonads ovaries 

 (Fig. 76, A, ovy) or spermaries as the case may be one 

 pair in each inter-radius. Each gonad has the form of a 

 bunch of grapes, being a much-lobed sac lined by epithelium 

 from which the ova or sperms are developed. It is con- 

 tinued into a tube or gonoduct, called spermiduct in the 

 male, oviduct (Ovd) in the female, which opens inter-radially 

 on the dorsal surface close to the bases of the arms. The 



