IX 



PHYLUM ECHINODERMATA 



373 



of the ring-vessel is connected with the axial sinus referred 

 to on p. 378. 



In the septum dividing the radial perihaamal sinus into two runs 

 a strand of a kind of gelatinous connective tissue containing many 

 leucocytes and perforated by irregular channels or lacuna : this 

 is the radial strand of the lacunar or haemal system. Like the 

 radial vessels of the perihaBmal 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. 313). The rows of am- 

 bulacral ossicles appear in 

 this view as ridges, the am- 

 bulacral ridges, one running 

 along the middle of the oral 

 surface of each arm to its 

 extremity, and extending in- 

 wards to the corresponding 

 angle of the mouth. At the 

 sides of each of these ridges 

 appear the rows of ampullse. 

 Within the pentagonal actino- 

 stome is a space, the peri- 

 stome, 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. 



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

 with a layer of ciliated epithelium, the epidermis or deric epi- 

 thelium (Fig. 310, Der Epithm), which is continued over the 

 various appendages and processes the tubercles and spines, the 

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

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

 mesoderm (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- 

 titions, developed in the outer of these two layers. Each ossicle 

 consists of a close network of calcareous rods. Between contiguous 

 ossicles extend bands of muscular fibres. 



The interior of the eoelome (Ccel.) or body-cavity is lined by a 

 ciliated epithelium, the ccelomic epithelium (Gael. Epithm.), which 

 not only covers the inner surface of the body-wall as the parietal 

 layer, but also forms an investment for the contained organs 

 the various parts of the alimentary eanal and its appendages, 

 the gonads, the madreporic canal, ampullae, etc. In addition 



FlQ. 309. Starfish. Lower part of a vertical 

 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. nerv. strand of deep nervous system ; 

 rad. bl. v. radial perihgemal 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 Cuenot.) 



