JX 



PHYLUM ECHINODERMATA 



deep nervous system has a central part in the form of a pentagon, 

 which in this case is double, surrounding the mouth. A third set 

 of nerve elements (the ccelomic nervous system) extend along the 

 roof of the arm superficial to the muscles. 



The two radial nerve-bands of the deep nervous system are 

 thickenings of the lining membrane of a space overlying the 

 radial nerve and underlying the radial ambulacral system. This 

 space (rad. bl. v.), extending, like the other parts that have been 

 mentioned, throughout the length of the arm, forms part of "a 

 system of channels which are usually regarded as constituting a 

 blood-vascular si/stem. This radial blood-vessel, as it is termed, is 

 divided longitudinally by a vertical septum (sept.} into two lateral 

 halves. Internally it com- 

 municates with an oral ring- 

 vessel surrounding the mouth 

 and likewise divided into two 

 by a septum. The inner 

 division of this ring-vessel 

 communicates with the cce- 

 lome ; the outer is connected 

 with the axial sinus referred 

 to below. 



Structure of the Disc. 

 -When the dorsal wall of 

 the central disc is dissected 

 away, the remainder of the 

 organs come into view (see 

 Fig. 283). The rows of am- 

 bulacral ossicles appear on 

 this view as ridges, the am- 

 bulacral rid yes, one running 



along the middle of the ventral surface of each arm to its ex- 

 tremity, and extending inwards to the corresponding angle of the 

 mouth. At the sides of each of these ridges appear the rows of 

 ampullae. Within the pentagonal actinostome is a space, the 

 peristome, covered with a soft integument, and in the centre of 

 this is a circular opening, the true month, the size of Avhich is 

 capable of being greatly increased or diminished. 



Body-wall and Coelome. The entire outer surface is covered 

 with a layer of ciliated epithelium, the epidermis or deric < j jn- 

 f. he Hum (Fig. 280, Der. Epithm.}, which is continued over the 

 various appendages and processes the tubercles and spines, the 

 pedicellarias, the dermal branchiae, and the tube-feet. Beneath 

 it is a network of nerve-fibrils with occasional nerve-cells. The 

 iiicsodcrm (Derm} of the wall of the body beneath this consists of 

 two layers, between which are a number of spaces : the ossicles (os.\ 

 are all, except the ambulacral ossicles and the inter-radial par- 



FlG. -/.'. Starfish. Lower part <>f a vertical 

 section through the arm, to show the structure 

 of the radial nerve and the position of the 

 deep nervous system and of the radial blm id- 

 vessels ; ii. nerv. strand "f deep nervous sys- 

 tem; rail. I>1. r. radial blood-vessel; /<!</. />!/<. 

 radial nerve; xijit. septum of radial blood- 

 vessel; ai-j-if. thickening of the septum some- 

 times taken for the radial blood-vessel. (After 

 Cut-not.) 



