114 ZOOLOGY. 



158. Digestion. The digestive organs are confined to 

 a very small space near the beak of the shell. This space 

 is separated from the rest of the interior cavity by a 

 membrane, in the centre of which the mouth is placed. 

 The mouth is furnished with two arms, which correspond 

 to the labial tentacles of the Lamellibranchiata. It was 

 formerly thought that the animal had the power of pro- 

 truding these arms, but it is now believed that they 

 remain permanently coiled up within the shell. They 

 are often three or four times the length of the shell, and 

 occupy a large portion of the interior cavity. They are 

 furnished on one side with a fringe of filaments, called 

 cirri (Lat. cirrus, a curl). These cirri are provided with 

 microscopic cilia, which produce a current in the water, 

 and thus bring a supply of food. 



The mouth leads to a gullet and stomach, which is 

 surrounded by a large liver. The intestine in some cases 

 terminates in a blind sac ; in others it is provided with 

 an anal opening. It is always completely shut off from 

 the cavity of the body. 



159. Circulation. The heart is in the form of a simple 

 sac, without any valve or partition, and is placed on the 

 dorsal side of the stomach. The course of the circulation 

 has not been fully ascertained. 



These animals possess what Professor Huxley has 

 termed an atrial (Lat. atrium, a hall) system, from its 

 resemblance to the atrial chamber in the Ascidians. It 

 consists of a series of canals which communicate with the 

 exterior by two or four organs, called false hearts, because 

 they were formerly believed to be connected with the 

 blood vascular circulation. The object of the system 

 seems to be to carry off excretions, and the products of 

 the generative organs. 



160. Respiration was formerly thought to be effected 

 solely by the mantle ; hence the name PalliobrancMata 

 (Lat. pallium, a cloak ; Gr. branchia, a gill) was given to 

 this class. The long fringed arms are now believed to 

 discharge the office of purifying the blood, 



