TYPE ECUINODERMA. 



569 



which enlarge into ten saclike reproductive organs, lying 

 in close contact with the walls of the genital bursse. The 

 ova and spermatozoa migrate from their point of origin in 

 the ovoid gland along the genital cords, which also contain 

 prolongations of the lacuuar tissue of the gland, and mature 

 in the reproductive pouches. During the spawning-time the 

 lobes of the reproductive organs protrude into the bursse, 

 pushing before them the thin walls of these pouches, and the 

 reproductive elements when mature burst through into the 

 cavities of the bursse, whence they make their way to the 

 exterior, or else, as in Amphiura squamata, undergo their de- 

 velopment in the pouches. 



From what has been said it may be seen that the genus 

 Astrophyton and its allies differ in many respects from the 



st 



FIG. 261. PLUTEUS LARVA OP EcJiinarachnius parma (after FEWKES). 

 a = oesophagus. TO = mouth. 



e = rudimeut of adult. s = calcareous skeleton. 



st = stomach. 



other Ophiuroids, having arms sometimes branched and 

 capable of being curled in over the oral surface, possessing 

 pedicellarise and lacking buccal shields, not to mention other 

 peculiarities. Consequently the class Ophiuroidea may be 

 regarded as consisting of two orders, the EUKYALIDA, includ- 

 ing Astrophyton, commonly known as the Basket-star, Tri- 

 chaster, in which the arms do not branch, and other similar 

 forms, and the OPHIUEIDA, which includes the other genera, 

 such as OpJiiotlirix, Ophioderma, Ophiolepis, Amphiura, etc. 



