XXVI REPRODUCTIVE ORGANS 315 



ling-vessel is a ring blood-vessel {¥\g. 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- 

 canal, 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 (J'ig. 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 nervous 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 gonoducf, 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 



