TYPE ECHINODERMA. 557 



relationship. In the walls of the stone-canal calcareous 

 matter is deposited, whence the name applied to it, and its 

 walls form projections extending into the lumen of the canal, 

 the surface for the ciliated epithelium being thus increased. 



The appendages of the oral ring are of two kinds, both 

 being situated in the interradii, that containing the stone-canal, 

 however, usually lacking any other appendage. In some 

 forms hollow saclike structures open into the ring by a 

 narrow neck, and are termed the Polian vesicles ; their walls 

 consist of connective tissue in which are situated muscle- 

 fibres, and their interior is lined by an epithelium which 

 appears to separate and give rise to the amoeboid cells of the 

 hydroccel fluid. The other kind of appendages occur generally 

 throughout the group and are known as Tiedemau's vesicles, 

 consisting of masses of hollow tubes arranged in pairs in one 

 or more of the interradii. The epithelium lining the walls of 

 these structures also seems to give rise to the amoeboid cells, 

 both kinds of organs being therefore comparable to lymphatic 

 glands, though the Poliau vesicles have also been regarded as 

 reservoirs for the hydroccel fluid. 



The mouth is situated at the centre of the oral surface of 

 the disk, and opens into a short oesophagus which, in some 

 forms, has connected with it ten glandular pouches. The 

 oasophagus opens into a usually capacious cardiac stomach 

 which is frequently lobed (Fig. 255, c), is eversible and pro- 

 vided with special muscles for its retraction. Above this 

 comes the pyloric stomach which gives rise to five radial 

 pouches, which soon branch into a pair of sacculated pouches 

 extending out into the arms, and being termed the radial 

 cfeca (I). From the pyloric stomach a short rectum passes 

 aborally, interradial caeca being sometimes found close to its 

 origin from the pyloric stomach, and opens upon the dorsal 

 surface. In a few forms, such as Luidia, Astropecten, and their 

 allies, the anus is wanting, but more usually it is present in 

 the region indicated. 



The epithelial nervous system consists of a plexus of 

 ganglion-cells and fibres imbedded in the ectoderm and cover- 

 ing the surface of the body, and of an oral ring and five radial 

 nerves (Fig. 254, JV) which, as in the Criuoids, are situated in 



