CRINOIDEA. 



409 



reproductive organs are situated beneath the skin in the grooves on the 

 ventral surface of the arms or pinnules. 



As the study of the fossil Crinoids is attended with considerable 

 difficulties, it may be well to give here a brief account of the general 

 anatomy and development of one of the " Feather-stars " (Comatula), 

 no other recent type of the group being readily obtainable for pur- 

 poses of investigation. All the known Crinoids are attached to 

 foreign bodies by a jointed stem or " column " in their young con- 

 dition ; but in the adult state they may either retain this stem of 

 attachment, and thus remain permanently fixed, or they may become 



Fig. 284. — Crinoidea. Antedon {Comatula) rosacea, a free Crinoid, viewed from its 

 dorsal or aboral aspect. 



detached from the stem and may lead a free existence. In accord- 

 ance with this, the Crinoids may be divided into the two groups of 

 the " Pedunculate Crinoids " and the " Free Crinoids." In the 

 Pedunculate Crinoids, where the stalk of attachment is permanently 

 retained (fig. 289), the animal may be compared with a Star-fish 

 turned upside down, the column springing from the centre of the 

 "dorsal" (or "abactinal") surface, while the "ventral" (or "ac- 



