226 



COMPARATIVE ANATOMY. 



dorsal side ; tlieh' products appear to pass into tlie coelomj whence 

 they reach the exterior by the narrow pores placed in the interradii 

 of the ventral surface (Fig. 102, g). In the ovoviviparous Ophiurida 

 this cleft may be seen to adapt itself to the necessary size. A chord- 

 like structure placed in each arm, and ramifying into the pinnulee, 

 forms the generative organs of Comatula. This genital chord is 

 surrounded by the blood-vessels just as are the branched tubes of 

 the Asterida. In the arms it remains sterile, and its products are 

 developed within the pinnulee from the walls of the chord. The 

 sperm passes out by the pores already described. 



§ 181. 



The generative glands, which in the Asteroida are arranged by 

 pairs in each radius, are converted into unpaired structures in the 



Echinoida ; this implies 

 further centralisation. 

 Their relation to the 

 primitive condition can 

 only be made out from 

 their interradial arrange- 

 ment, so that each organ 

 may be regarded as 

 foi'med of two radial 

 ones. They form greatly 

 ramified glands, which 

 generally project far into 

 the coelom (Fig. 116, g), 

 and open at the genital 

 plates (Fig. 103, g). In 

 the Spatangidfe one of 

 the five genital glands, 

 which are the typical set 

 for the Echinoida, is 

 atrophied, and at the 

 same time the genital 

 plate, which was the madreporic plate also, becomes the madreporic 

 plate only. 



In the Holothuroida they are still more reduced. The testis or 

 ovary forms a tuft of richly-branched tubes, which unite at a 

 common duct (Fig. 113, 0) ; this opens near the mouth, and 

 ordinarily between the tentacles. This indicates a relation to the 

 radii; the organs, which in other forms are broken up, are here united 

 into one, and the higher grade of organisation, already seen in the 

 Echinoida, is still present, as is shown by the presence of an 

 excretory duct. 



In the Synapta3 the organs are hermaphrodite, and foi'med 

 on the Holothurian type. The various tubular glands are united at 

 a common excretory duct, which opens to the exterior above the 



Fig. 116. Generative organs of au Echinns. 



Eather more than the ventral half of the shell has 



been removed, a AmpullEe of the ambulacra. 



i Hind-gut. g Ovarian acini. 



