GENEEATIVE OEGAXS OF ECHIXODEEMA. 



225 



system is simple in structure, the excretory ducts are not com 

 plicated, and there is no intromittent 

 organ, so that the surrounding* water 

 is the medium of communication in 

 impregnation. On the whole there 

 is a great resemblance to the corre- 

 sponding structures found in Vermes. 

 The lowest stages, both as to num- 

 ber, arrangement, and as to the more 

 special characters of the organs, are 

 seen in the Asteroi'cla. The testes or 

 ovaries are tubular or lobate glan- 

 dular canals, which in some forms are 

 arranged in two rows, and disposed 

 conformably with the metamerism of 

 the arms (Ophidiaster, Ai'chaster). 

 In others there are only two groups 

 in each arm, which may extend along 

 the whole cavity of the arm (this is 

 the case even in Brisinga), or may 

 be limited to the interradial space 

 merely (Fig. 108, ;/). A comparison 

 of these characters shows us, that there 

 is a gradual reduction in the number 

 of the generative glands, which corre- 

 sponds with the gradual centralisation 

 of the organism, which we have 



Fig. 114. Generative organs of an 

 Ophiurid (Opliioderma lougicauda). 

 The dorsal integument and the 

 digestive organs have been removed. 

 ';• Arms, g Ovarian acini. 



ah-eady seen to occur in the Asteroida. 

 In the aproctous forms the tubes have 

 no excretory orifices, and the genera- 

 tive products are passed into the body- 

 cavity. We do not yet know how 

 they reach the exterior. In other 

 Asteroida the generative glands open 

 to the exterior by special plates, dis- 

 tinguished by their fine pores (cribri- 

 form plates), and placed in the dorsal 

 interradii ; or they have a single duct 

 with a narrow orifice (Pteraster). 

 Each organ is surrounded by a blood 

 sinus, which envelopes the separate 

 lobes and lobules. The generative 

 products pass into this sinus and are 

 not evacuated directly. 



The structure and arrangement of 

 the generative organs of the Ophiurida 

 is the same as in the Asteroida. There is only one example of 

 hermaphroditism (Ophiura squamata). The generative glands (Fig. 

 114, (/) are arranged by pairs in each interradial space, and on the 



Fig. 115. Transverse section 

 through the pinnula of a sexually 

 mature Comatula (Ajitedon Esch- 

 richti). The dorsal surface is 

 turned upv^ards, and the ventral 

 downwards, 'p Tentacle, g Lumen 

 of the genital chord, w Water- 

 vessel, with lateral branches into 

 the tentacles, n Nerve - chord. 

 h Blood-vessel on it. c g Canal 

 around the genital chord, c d Dorsal, 

 c V Ventral canal. All three com- 

 niunicating with the coelom (after 

 H. Ludwig). 



