Mr. Robert Garner's Malacological Notes. 367 



by Cuvier to distinguish the separate families. They are 

 generally protected by a shell, and are of course en liaison 

 with the heart, as the latter is, more or less, with the alimen- 

 tary canal ; indeed we should have before observed that in 

 the Lamellibranchiates the ventricle is commonly traversed by 

 the rectum, this arrangement being either an advance upon 

 the disposition seen in still lower animal forms, where the 

 intestine is enveloped by the general reservoir of the blood, 

 or, as has been thought, due to the young bivalve being the 

 result of the union of twin embryos. It is probable, as may 

 be traced in their vascular arrangement, that the branchise in 

 mollusks are normally four in number, as we see them in 

 bivalves and again in Nautilus ; the two of one side, how- 

 ever, are combined into a single one in most Gastropoda, 

 and in some Arcce^ Anafina, Holemya, and other bivalves ; 

 or one only, altogether (homologically duplex however), may 

 exist, as in other Gastropods, its fellow of the opposite side 

 being more or less undeveloped. Are the pair of branchiae 

 found in llie dibranchiate Cephalopoda the representatives of 

 the four branchiaj of the tetrabranchiate Nautilus ? or have we 

 the rudiments of the wanting pair, or simply of the corre- 

 sponding cardiac parts, in the anomalous appendages of the 

 lateral hearts, which, however, are wanting or little developed 

 in Octopus^ The monobranchiate ^/)/j/s?'a has an aortal appen- 

 dage. Some of the Gastropoda have pulmonary sacs instead 

 of branchige, and others [Ampullaria and perhaps some lit- 

 toral species) have both*. 



The position of the branchiee in Patella and Chiton (Cyclo- 

 branchiata) is analogous to that in the bivalves, to which 

 mollusks these Gastropods form the natural transition ; but the 

 ventricle of the heart has not the intimate connexion with the 

 rectum, though both heart and rectum are situated at the 

 posterior extremity of the body in the latter genus. Not so 

 in those allied genera where the branchiaj have ascended wholly 

 or partially above the neck (Scutibranchiata) — Ftssurella^Emar- 

 ginula, and Haliotis] for here the ventricle and rectum are in 

 union, as in the bivalve. In Haliotis and Siga7'etus one 

 branchial plume is commonly less than its fellow ; and in 

 Haliotis the inequilateral composition of the shell is indicated 

 by the row of foramina. In Cali/j^trcea and its congeners the 

 smaller of tlie branchi^ has disappeared ; and in this last case, 

 probably, the shell is correspondingly the expanded representa- 



* The circuuipedal friiige of Patella has doubtless a branchial function ; 

 but we do not deny that the animal, when exposed by the recedino; tide 

 may take air into the supradorsal cavity, tliough this rather appears to 

 be the renal organ. 



