PHYLUM ECHINODERMATA 



381 



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

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

 thelium (Fig. 305, Der. Epithm}, which is continued over the 

 various appendages and processes the tubercles aud 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 (ps) 



FIG. 305. Diagrammatic sections of a Starfish. A, vertical section passing on the right through 

 a radius, on the left through an inter-radius. The off-side of the ambulacral groove with the 

 tube-feet (T. F.) and ampullae (Amp.) are shown in perspective. B, transverse section through 

 an arm. The ectoderm is coarsely dotted, the nervous system finely dotted, the ectoderm 

 radially striated, the mesoderm evenly shaded, the ossicles of the skeleton black, and the 

 coelomic epithelium represented by a beaded line. Amb. ox. ambulacral ossicles; Amp. am- 

 pulla? ; An. anus ; C. Amb. V. circular ambulacral vessel : C. B. V. septum of ring perihaemal ; 

 vessel ; Cd. C<K. cardiac caeca ; CM. cojlome ; Ctel. Epithm. ccelomic epithelium ; Der. Epithm. 

 deric epithelium ; Derm, mesoderm ; Ent. Epthm. enteric epithelium : Int. ca>. intestinal caeca. 

 Mdpr. madreporite ; Mes. mesentery ; Mill, mouth ; Nv. R. nerve-ring ; oc. eye ; os. ossicles of 

 body-wall ; Ovd. oviduct ; Fed. pedicellariae ; ph perihaemal spaces ; Pyl. ccec. pyloric caeca ; 

 Rod. amb. v. radial arnbulacra.1 vessel ; Rod. B. V. points to septum in the radial perihaemal 

 vessel ; Rad. Nr. radial nerve ; Resp. ca>. dermal branchiae ; St. stomach ; St. c. stone-canal ; 

 t. tentacle; T. F. tube-feet. (From Parker's Biology.) 



are all, except the ambulacral ossicles and the iriter-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 ccelome (Ccel.) or body-cavity is lined by a 

 ciliated epithelium, the coelomic epithelium (Cosl. 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 



