VIII EC'HINODERMATA—ALIMENTARY CANAL 481 



mesenteries are broken through in such a manner as to form elegant arcades. In the 

 Spalangaidca, however, the different coils of the canal are further connected together 

 inter sc by mesenteries which are not perforated. 



Unicellular glands of various kinds are found, chiefly in the epithelium of the 

 first section of the intestine. In the Spatangoidea, in the commencement of the first 

 spiral, there are multicellular flaskj-shaped glands ; these lie in the connective tissue 

 layer, the neck-like duct alone opening into the lumen of the alimentary canal. 



The aeeessopy intestine (siphon), which occurs in nearly all 

 Echinoidea, deserves special attention. Near the commencement of 

 the first spiral, the siphon branches off from the main intestine as a 

 narrow tube, which again enters the intestine at the end of that spiral, 

 to which it thus belongs. The siphon always runs along the inner 

 side of the main intestine (that turned to the principal axis of the 

 body). In regular Echinoids, it follows the main intestine in its 

 course ; in the Clypeastroida, on the contrary, its course is somewhat 

 shortened. In the Spatangoida, the first part of its course is shorter 

 than that of the main intestine, while the rest follows the coils of the 

 latter. 



The Cidaraida {Dorocidaris pajnllata) have no distinct siphon, but 

 it is very probable that this organ is here represented by a longitu- 

 dinal furrow bordered by two folds, which furrow is either not yet, or 

 no longer, shut off from the lumen of the intestine. This furrow 

 occurs in the same region of the intestine as the siphon, and also on 

 the axial side of the canal. 



In the Spatangoid genera Brissus, Brissopsis, and Sdiizaster a 

 second siphon has been discovered, running parallel to the intestine. 



The structure of the siphon resembles, in essential points, that of 

 the main intestine. It has been conjectured that it, like the accessory 

 intestine of certain worms, subserves intestinal respiration. 



D. Cpinoidea. 



In this class the alimentary canal is tubular. It descends from 

 the mouth into the calyx, coiling in the direction of the hands of a 

 clock (when the body is viewed from the oral side). From the base 

 of the calyx it again ascends, continuing the same curve, towards the 

 tegmen calycis, and here enters the anal cone in the anal interradius ; 

 it then runs through the anal cone, opening outward at its tip through 

 the anus. 



During its course through the calyx, the intestine makes one 

 complete eoil round the principal axis (Fig. 382, B, p. 475). The 

 alimentary canal of Adinometra affords a striking exception to this 

 rule, forming, in the same direction as in other Crinoids, as many as 

 four coils (Fig. 382, 0). It may be remembered that Adinometra is 

 also distinguished from all other Crinoids by the eccentric position of 

 the mouth in the tegmen calycis. 



The section of the intestine which lies at the bottom of the calyx 



VOL. II 2 I 



