five arms '). The mouth situated at the ventral side of the 

 animal, leads by a narrow neck, called the ..oesophagus" ^), 

 into a voluminous sac. sometimes divided in two parts by a 

 circular sphincter, called the ..stomach". In front of the mouth 

 we find the proximal members of the adambulacral series of 

 sceletal parts, the so-called mouth-spines. The stomach is 

 produced into five ..stomach pouches", sometimes forked, pro- 

 jecting into the arms. These pouches and therewith the whole 

 stomach can be retracted by means of two very thin and long 

 muscles, lying ventrally and lengthwise on the ambulacral 

 groove and fixed to the median ridge of the ray. The stomach 

 pouches are continued by one or two ducts — in the place of 

 bifurcation there is considerable variation — running out into the 

 arms. Here they form the so-called pyloric coeca. accumula- 

 tions of small pouches the little ducts of which all come out 

 on the chief duct. These radial sacs are dark greenish yellow 

 in color and attached to the dorsal wall of the coelomic cavity 

 by suspensory bands of membrane, called mesenteries, simple 

 continuations of the peritoneal lining of the coelom. Beyond 

 the pyloric coeca the alimentary tract is continued as a slender 

 , .rectum", ending in the anus. Both are thin, relatively small 

 and not of great importance ; they are absent in the Astropec- 

 tinidae and the Porcellanasteridae. The rectum gives off 2 — 5 

 pouches, sometimes forked, which are called rectal or interradial 

 coeca. They mostly contain a dark, brown fluid ; their function 

 will be discussed in the chapter on excretion, since they pro- 

 bably have excretory function. The radial coeca of the stomach 

 are very constant in structure and number; the rectal coeca 

 vary largely. In Asterias we find two simple fans, in the 

 Pentacerotidae there are five forked coeca, in Asterina five 

 simple sacs and in the Echinasteridae and Astropectinidae one 

 five-lobed coecum. 



The histology of the digestive tract of the starfishes is in 

 its general features identical with that of all Echinoderms. We 

 can distinguish (from outside to inside) : 1 . an endothelial layer, 

 a simple continuation of the peritoneal lining of the coelomic 

 cavity, showing a. mucus cells and b. simple ciliated endothelial 

 cells, 2. a thin layer of connective tissue. 3. a layer of muscles 

 and 4. the epithelium which consists of long cylindrical cells, 

 ciliated in most parts, with nerve fibres in their basal part. 

 The mouth is opened and shut by means of an internal circular 

 sphincter and of external radial dilatators ; in other parts the 

 longitudinal muscles lie at the inside, the circular ones are 

 external. In the stomach we find a very strong musculature ; 



') If more arras are present (their number can be as high as 25. e. g. in 

 the Hehasteridae and Brisingidae), their number of course follows that of the arms. 



') Sometimes (in Echinaster and Cribrella f. i.) the oesophagus shows ten 

 pouches, five radial and live interradial ones. 



