NO. 7 SEA-LILIES AND FEATHER-STARS — CLARK 9 



its central axis from which the partitions dividing it into five sections 

 arise is continued irregularly upward to the vicinity of the mouth as 

 the so-called axial organ, a cord of connective tissue including a 

 number of open spaces or lacunse. Five slender prolongations, one 

 from each of the chambers of the chambered organ, accompany the 

 axial organ for some distance, ending blindly, and each cirrus 

 includes a median vessel from the same organ. 



Except at the point where the axial organ leaves it the chambered 

 organ is completely encased in a mass of nerve fibrillae from which the 

 large dorsal nerves of the arms and pinnules and their various 

 derivatives as well as the cirrus nerves arise (fig. 9) ; while over this 

 nervous envelope and along the nerve cords lie multitudes of wander- 

 ing cells which play an important part in the regeneration of lost 

 members. The envelope of the chambered organ and its derivatives, 

 including also a closely associated nerve ring about the mouth which 

 sends off very numerous branches including two cords running along 

 the ventral surface of each arm, form the chief nervous system of the 

 crinoids, but beneath the ambulacral grooves of the disc, arms and 

 pinnules there is second quite independent nervous system consisting 

 of a continuous thin layer of nerve fibrillae. 



The water vascular system of the crinoids consists of a ring canal 

 about the mouth and vessels radiating out from it under each of the 

 ambulacral grooves which they follow in their course to the pinnule 

 tips, sending off branches into the delicate tentacles. Attached to 

 the ring canal is a row of little tubes which open into the body cavity, 

 and communication between the body cavity and the sea water outside 

 the body is maintained by a number of calyx pores (5 in Rhisocrinus, 

 .1500 in Antedon mediterranean and still more in larger species) which 

 pierce the body wall. 



The blood vascular system of the crinoids is very highly developed, 

 though the blood vessels are nothing more than intercommunicating 

 cavities or gaps in the connective tissue of the mesenteries, bands and 

 cords which in all directions traverse the body wall, the walls of the 

 digestive tube, the axial organ, etc. 



The so-called genital cord forms an irregular pentagon about the 

 mouth from which five branches are given off, these running beneath 

 the water tubes into the arms and pinnules ; but it is only in certain 

 of the lower pinnules in most comatulids or at the pinnule bases and 

 rarely in the arms in the stalked types that the sexual products are 

 developed. It has already been mentioned that in the short posterior 

 arms of certain of the Comasteridae the ambulacral grooves are absent ; 

 when this occurs the ambulacral nerves, water vessels and tentacles 

 are also absent, but the genital cord is here especially developed. 



