4 CRETACEOUS PELECYPODA 



wliicli pewiits the great expansibility of the muscle ; its position is regulatctl 

 by certain muscles attached internally to the shell. There are generally two pairs 

 of tliese pedal muscles present, one anteriorly and the other posteriorly, each 

 susjjended above one of the muscular scars. It does not appear to have been 

 noticed that the byssus of the Pelecypoda is secreted exactly at the same place 

 where in the Gastropoda the operculum is situated, and although both these 

 organs are very differently shaped and developed, and serve entirely different 

 purposes, there exists as to position an unquestionable homology between them. 



The food of the Pelecypoda consists of the most minute particles, in a great 

 measure of Infusoria wduch are brought into the cavity of the body with the 

 water required for respiration. The current of water passes from the siphons 

 anteriorly towards the mouth, and it appears to be the chief function of the labial 

 iialps to direct or convey siicli particles as may serve for food to the mouth, the 

 lips of which are fleshy, but not hardened. A kind of salivary glands has been 

 observed in Teredo and allied forms, but it appears to me that these glands are 

 of common occurrence in most of the Pelecypoda,* though as yet apparently not 

 recorded. They consist of a pair of usually very small globular or sub-cylindrical 

 masses which lead by a thin canal into the muscular oesophagus, immediately 

 l)ehind the mouth. The stomach is, as usually in other MoUusca, enveloped in 

 the liver, and so is also a portion of the alimentary canal which does not attain 

 any considerable length ; the former often has in front an appendage which is 

 filled with the so-called crystal-style, and which I have also often observed in many 

 Gastropods. The rectum passes through the heart and terminates behind the 

 posterior retractile muscle, where the ejecta pass into the exhalant siphonal opening, 

 or freely between the mantle at the place where the water issues from the body. 

 Besides the liver, the only organ of a peculiar secretive character is a pair of so- 

 called Boj anus' -glands ; they are situated posteriorly close to the heart, a little in 

 front and above the posterior retractile muscle. These glands are of a spongy appear- 

 ance, either of yellowish or violet color, and are partially connected with the heart, 

 partially with the vascular system direct, often admitting water to the pericardium 

 or the heart. It seems certain that to some extent at least these Bojanus'-glands 

 represent the spleen and kidneys ; their internal canals either terminate separately 

 close to the generative organs, or together with them in the same opening. 



The circulation of the blood has as its central organ a heart which receives 

 from the gills the oxidised blood, though this becomes partially mixed with venous 

 blood just before it enters the heart. The greater part of the venous blood, 

 however, coming from the foot, the mantle, &c., collects either in the Bojanus'- 

 glands, or returns direct to the auricles of the heart. This impurely oxidised, 

 mixed blood issues from the heart in two principal branches - arteries - one leading 

 to the intestines and towards the mouth, the other to the foot ; from the former 

 arc supplied by repeated branchings the mantle and the palps, from the latter 

 the generative organs. 



* 1 hope to be able to publish some further details on this anatomical subject. 



