IX 



PHYLUM ECHINODERMATA 



383 



looked upon as digestive glands. It is found by experimenting 

 with this digestive fluid that it has an action on food-matters 

 similar to that exerted by the secretion of the pancreas in the 

 Vertebrata, converting starch into sugar, proteids into peptones, 

 and bringing about the emulsification of fats. While the pouches 

 of the cardiac division of the stomach are attached to the oral 

 wall of the body, the pyloric caeca are connected with the aboral 

 wall. From the short intestine are given off inter-radially two 

 hollow appendages, the intestinal caeca (Figs. 306 and 308, int. mc), 

 each with several short branches of irregular shape. 



Ambulacral System. — Running downwards from the madre- 

 porite to near the border of the mouth is an S-shaped cylinder, 

 the madreporic or stone-canal (Figs. 303, m'. 310, mad. can). The 

 walls of this canal are supported by a series of calcareous rings, 

 and projecting into it is a ridge which bifurcates to form two 

 spirally rolled lamellse occupying a considerable part of the lumen of 

 the canal. In some Starfishes, such as Astropecten (Fig. 307), the 

 internal structure is more com- 

 plicated owing to the branching 

 of the lamellEe. The interior of 

 the madreporic canal communi- 

 cates above with the exterior 

 through the grooves of the madre- 

 porite. At the bottom of each 

 of the grooves is a row of pores 

 leading into a sac, the ampulla, 

 which in turn leads into the 

 madreporic canal. Below, the 

 latter opens into a wide, five- 

 sided, ring-like canal, the ring- 

 vessel of the ambulacral system. From this are given off the five 

 radial ambulacral vessels, passing to the extremities of the arms. 

 From the pentagonal canal are given off also in most Starfishes, 

 but not in Asterias, a series of five pairs of appendages, the Polian 

 vesicles (Fig. 303, ap ; Fig. 308, 'pol. ves) — pear-shaped, thin-walled 

 bladders with long narrow necks — which are placed inter-radially. 

 At the sides of the neck of each Polian vesicle (except in the 

 inter-radius containing the madreporic canal, where there is one 

 on one side only) project inwards a pair of little rounded glandular 

 bodies, the racemose or Tiedemann's vesicles (Fig. 309, T), the cavity 

 in the interior of each of which, opening into the ring-vessel, is 

 divided into a number of chambers. 



The various parts of the ambulacral system of vessels have a 

 muscular wall and an internal lining epithelium in addition to the 

 coverings which they may derive, according to their situation, 

 either from the external epidermis or the internal coelomic epi- 

 thelium. The muscular layer is most strongly developed on the 



Fig. 307. — Transverse section through the 

 madreporic canal of a Starfisll {Astro- 

 pecten). (From Gegenbaur, after Teuscher.) 



