THE ALIMENTARY CANAL. 



The alimentary canal (Plate L Fig. 7) arises from a stomadaeuni which forms 

 the epithelium of the pharynx, esophagus, and probably of the stomach, and from 

 a second anlage which forms the caecum , intestine , rectum , inksac , liver and 

 pancreas and whose origin has been a mooted point. This anlage is first clearly 

 seen as a small epithelial cap whose edges rest upon the yolk mass and whose 

 cavity is Ijounded below by the yolk. Watase believes this to be a proctodaeum 

 but it now seems quite clear that a proctodaeum is not formed. Apart from 

 his view . it is generally agreed that the cells forming this cap migrate 

 singly from the ectoderm as do all the entodermal , mesodermal (and yolk?) 

 cells of the embryo and later arrange themselves in an epithelium. Hence 

 we believe that the whole discussion is a essentially a matter of names. To 

 BoBRETZKY. it is mesodermal: to Paussek and Teichman , it is mesentodermal 

 and becomes the definitive entoderm : while to Vialleton and Korschett , it 

 is entodermal. 



The glands of the gut are a median and a paii of lateral salivary glands, 

 a pair of compound glands which form the pancreas and liver, and the inksac. 



The Pharynx. The mouth (Plate 111. Fig. 17) is surrounded by the buccal 

 and peristomial membranes. The former is a thin circular sheet attached to the 

 bases of the arms and extending forward around the latter. The buccal membrane 

 is supported by seven muscular columns which form the projecting points of the 

 scalloped free edge of the membrane. The inner surface of each column bears 

 ten or twelve stalked suckers arranged in two rows. The columns are attached 

 to the muscular ring at the bases of the arms as follows: - a median column 

 between the bases of the first pair of arms, one between each first and second 

 arm, one between each tliird and fifth arm and two side by side between the 

 fifth arms. 



The buccal membrane of the female has a horseshoe-shaped depression on 

 its inner surface below the mouth. The spermatophores are closely packed on 

 end in this depression and they form a white spot which, when present, is 

 a convenient and positive mark of the female. 



The peristomial membrane is a tubular fold of integument, arising in the 

 angle between the base of the buccal membrane and the pharynx and contaming 



