364 



ECHINODERMATA 



[CH. 



8 - 



the Crinoid possesses another and much more important nervous 

 system. From the body-cavity five canals are given off which 

 penetrate the stalk. These canals swell up in the substance of the 

 centro-dorsal ossicle into chambers, and in the permanently stalked 

 forms like Rhizocrinm or Pentacrinus they form similar chambers 

 wherever the stalk bears cirri. The coelomic cells which form the 

 walls of these canals develop great masses of nervous fibrillae. In 

 Antedon of course only the five uppermost chambers remain when 

 the stalk disappears they are termed collectively the chambered 



organ and the nervous lining of 

 these constitutes a kind of brain 

 (Fig. 170). This "brain" is sepa- 

 rated from the body-cavity of 

 the calyx by a sh elf-like fold 

 strengthened by a calcareous plate 

 called the rosette, which repre- 

 sents a circle of five plates alter- 

 nating with the columns of radials 

 clearly seen in more primitive 

 Crinoids. From this brain cords 

 go off to the cirri, and five great 

 cords run upwards, perforating the 

 radial rows of plates and eventually 

 bifurcating pass into the arms, per- 

 forating the plates which form the 

 skeleton of the latter. These cords 

 are at first tubular outgrowths 

 from the brain, the cells forming 

 the walls of which become con- 

 verted into nervous matter. It 



has been experimentally proved that it is this nervous system which 

 controls the muscles moving the arms, and that if the whole soft 

 part of the disc including the ambulacral nervous system be re- 

 moved the animal swims just as well as before. 



The organs of sex are rounded masses found in the pinnules 

 and are really, as in Asteroidea, Ophiuroidea and Echinoidea, 

 swellings on branches of a genital rachis. There is also a genital 

 stolon, which however has no connection with any of the stone- 

 canals, but rises from the rachis to the centre of the dorsal wall 

 of the coelom. The young are carried on the pinnules for some 

 time and have a very short free-swimming life, very soon settling 



FIG. 172. Ventral view of a larva of 

 a Holothurian taken at Marseilles 

 x about 100. From Job. Miiller. 



1. Mouth. 2. Oesophagus. 



3. Stomach. 4. Kectum. 



5. Anus. 6. Coe.lomic sac. 



7. Budiment of water- vascular system . 



8. Ciliated band. 



