380 



ZOOLOGY 



{d. new.) also of a nervous character. These more deeply placed 

 nerve-bands are the radial parts of the deep nervous system : like 

 the epidermal, the deej) 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 cmlomic nervous 

 system) extends 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 {rod. hi. v), extending, like the other parts that have been 

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

 system of channels, the perihcemal system, which have been regarded 

 as constituting a blood- vascular system. This radial 2ierihwmal 



vessel or sinus, as it is termed, 

 is divided longitudinally by 

 a vertical septum {sept.) into 

 two lateral halves. Internally 

 it communicates with an oral 

 ring-vessel surrounding the 

 mouth and likewise divided 

 into two by a septum. The 

 inner division of the ring- 

 vessel is connected with the 

 axial sinus referred to on 

 p. 384. 



In the septum dividing 

 the radial perihsemal sinus 

 into two runs a strand of a 

 kind of gelatinous connective 

 tissue containing many leuco- 

 cytes and perforated by ir- 

 regular channels or laeunce : 

 this is the radial strand of the lacunar or limmal system. Like the 

 radial vessels of the periheemal system, the radial strands of the 

 lacunar system are connected internally with an oral ring. 



Structure of the Disc. — When the aboral wall of the central 

 disc is dissected away, the remainder of the organs come into view 

 (see Fig. 308). The rows of ambulacral ossicles appear in this 

 view as ridges, the amhiclacral ridges, one running along the 

 middle of the oral surface of each arm to its extremity, 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 mouth, the size of which is capable of 

 being greatly increased or diminished. 



Fig. 304.— Starfisb. Lower part of a Tertioal 

 section through the arm, to show the structure 

 of the radial nerve and the position of the 

 deep nervous system and radial perihsemal 

 vessels, d. new, straiid of deep nervous system ; 

 rad. bl. V. radial perih»mal vessel ; rad. nerv. 

 radial nerve ; sept, septum of radial peri- 

 hsemal vessel ; sept', radial lacunar strand of the 

 haemal system (here represented as solid). 

 (After Cutoot.) 



