viii ECHINODERMATA MORPHOLOGY OF SKELETON 377 



(>-) The Nerve Canals of the Arms and of the Apical Capsule. 



(Figs. 327-330). 



The skeletal joints of each arm (the brachials) are perforated by 

 an axial canal, which is continued to the tip of the arm, and into the 

 pinnulae, Where the arms fork or branch in various ways, the axial 

 canal does the same. It contains nerve strands, and may thus be 

 suitably considered as a nerve canal. This canal is continued right into 

 the base of the dorsal cup, perforating the radials, basals, and also the 

 infrabasals, when present. All the nerve canals, and the nerves within 

 them, converge towards the apex of the calyx, where, either in the base 

 of the dorsal cup itself (surrounded by the basals in stalked Crinoids), or 

 partly enclosed within the centrodorsal (Antedonulce), lies the central 

 organ of this nervous system, which surrounds the so-called five- 

 chambered sinus in the shape of a cup or capsule. From this point 

 the already mentioned central, or axial, canal runs through all the 

 ossicles of the stem, giving off lateral branches to the cirri. 



The nerve strands arise from this apical central nervous system at five interradial 

 points. The five interradial strands fork either in the basals or in the radials. 

 Within the radials each branch of a strand becomes connected with a branch of the 

 neighbouring strand, and from these radially arranged points of junction the radial 

 nerve strands originate which pass from the radials into the costals, a'nd are 

 continued into the ossicles of the arms and of their branchings. Within the circle 

 of radials there is, besides, a circular commissure between the radiating nerve 

 strands ; the following diagrams illustrate the courses taken by these. 



In the Pentacrinida.', Encrinidce, and Antedonidce, where the nerve strands 

 divide in the first axillaries, there is a peculiar chiasma nervorum brachialium, 

 which is shown in the diagrams. 



In Encrinus, and it is said also in Pcntacrinus, the nerve strands which run 

 through the ossicles of the arms are double. But whereas, in Encrinus, they run 

 separately, and are enclosed in separate canals, in Pcntacrinus they lie in a common 

 canal. 



Many palaeozoic Crinoids, and above all the Camcrata (with the exception of the 

 Crotalocrinoids), appear to have no differentiated nerve canals in the arms. 



(/) The Water Pores. 



In the Canaliculata (e.g. Pentacrinus, AuMm, Adi no metro) the 

 tegmen calycis, whether naked or plated, is perforated by so-called 

 water pores, whose significance will be discussed more in detail 

 later on. 



If the tegmen is plated, many or all of the plates of the interambulacral areas 

 are perforated by one or more such pores. In Pcntacrinus dccorus, as many as twenty 

 pores are found on one plate. The total number of pores varies greatly in different 

 genera and species. In Antcdon rosacea it has been estimated at 1500, and in other 

 forms may be even greater. The pores are usually limited to the tegmen, and are 

 least plentiful in the posterior interradius. They may, however, also occur on the 



