DIGESTIVE ORGANS. 105 



has the power of protracting and retracting at pleasure. In the 

 Thecosomata, those pteropods having a shell, the head is indis- 

 tinct and tentacles radimentar}^, and the mouth is situated in a 

 cavity formed by the union • of the two fins. There ai'e two 

 corneous jaws and a lingual ribbon. 



In all pteropods the rest of the digestive apparatus is similar 

 and simple in structure. There is a pair of salivary glands ; the 

 membranously walled stomach is encircled by the liver which 

 adheres and opens into it by several orifices ; * the short intestine 

 opens by an anus on the right side of the body. The kidney is 

 represented by a contractile sac communicating on the one side 

 with the sinus of the pericardium, and on the other with the 

 exterior. It does not appear to be secretory in function, its 

 principal object being the introduction of water to the peri- 

 cardium. 



Scaphopoda (xiv, tl ; iii, 43). The Dentaliidse are animal 

 feeders, devouring foraminifera. They have no specialized head, 

 but the mouth is surrounded by several labial palpi, and at its 

 base are numerous very extensible filaments, covered by vibratile 

 cilia, and terminated by a sucker. There is also a lingual ribbon. 

 The intestine is short, and the rectum traverses the sanguineous 

 pocket which represents the heart. The kidneys are paired, 

 opening on the right and left of the anus. 



Pelecypoda or Lamellihranchiata (xiv, tS). The digestive 

 organs are much more simplified in the bivalve shell-fish than in 

 the Gastropoda. The mouth is at the anterior part of the body 

 between two pairs of labial palpi (ix, 100 ; iii, 50, 52), appearing 

 like accessory branchiae and striated upon one face. These lips 

 are equal in dimension, triangular, and var^dng much in size in 

 the different genera : in Tellina even as large as the branchiae, 

 although they are generallj^ much smaller. Mostly simple in 

 outline, those of the Pectinidte are arborescently lobulated. No 

 apparatus exists in connection with the mouth for the commi- 

 nution of the food, neither tongue, jaws nor muscular pharynx ; 

 salivary glands are also wanting. From the mouth a short 

 oesophagus opens into the stomach, and the long intestine, which 

 is usually much convoluted, is terminated by a rectum opening- 

 above the posterior adductor muscle (xxii, 65^. Mr. Ryder f 

 finds the intestine in the American oyster, to have but one 

 complete turn upon itself, and in the course of its (dorsal) 

 flexure, to pass almost directly over the mouth, and to be pro- 

 vided with a pair of internal longitudinal folds. Except in 

 Ostrea, Teredo, etc., the rectum traverses the ventricle of the 



* In Hyalsea. there is a crop and gizzard armed with corneous plates ; 

 the liver is not adherent. , 



t Am. Naturalist, 674, 1880. 



