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 perihaemal sinus into two runs 

 a strand of a kind of gelatinous connective tissue containing many 

 leucocytes and perforated by irregular channels or lacunae : this 

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

 radial vessels of the perihaemal system, the radial strands of the 

 lacunar system are connected internally with an oral ring. 



Structure of the Disc. 



-When the aboral wall of '- - v'-- : r r"-- 

 the central disc is dissected .- .-:;;"->;/ '.- ;;*', .* v .;--".;'.: ;; " 1 ,>;',' 



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 , x 



surface of each arm to its ^HJP ^w//r/v ' 



extremity, and extending in- FIG. SOO. Starfish. Lower part of a vertical 

 nrorrla fr> f>a nnrrpcnrmrlino- section through the arm, to show the structure 



wards to the corresponding of the ra(Ual nerve and the position of the 



angle of the mOUth. At the deep nervous system and radial perihy.nal 

 . -P . P . , vessels, d. nerv. strand of deep nervous system ; 

 Sides of each of these ridges rad. bl. v. radial perihosmal vessel ; rail. nerv. 

 ,1 e i? radial nerve ; si-itt. septum of radial peri- 

 appear tne rOWS OI ampullae. ha-mal vessel ; sept', radial lacunar strand of 



Within the pentagonal actino- (IfteTculnot 8 ) 6111 (here represente 

 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 Coslome. 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 

 pedicellariae, 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 ecelome (Gael.} or body-cavity is lined by a 

 ciliated epithelium, the ccelomic epithelium (Ccel. 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 canal and its appendages, 

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



