216 Mr. J. E. Gray on Prof. Loven^s Homologies of Mollusca. 



when the animal and shell are fully developed, is not formed until 

 the animal has arrived at its full growth, and indeed is of so 

 little importance in the functions of the animal, that it is some- 

 times not permanently present, but only generated when required; 

 and it is often found in some genera of a family and not in others, 

 and is sometimes even present and absent in the species of the 

 same natural genus. 



On the contrary, the operculum, like the shell, is always to 

 be observed on the very young animal in the egg, and is always 

 permanently attached to, and indeed forms as much an integral 

 part of, the animal as the shell itself. I may further observe, 

 that if the byssus of the bivalve and operculum of the univalve 

 were homologous, they would not be found in the same animal, 

 yet many operculated univalve and some inoperculated gastero- 

 pods have the faculty of forming a byssus which is secreted by 

 the foot ; and this byssus appears to be the exact homologue of 

 the byssus of the bivalve. This byssus has been observed in 

 the genera Rissoa, Cerithium, Littorina and Litiopa, all of which 

 are provided with a well-developed operculum. 



I may also remark, that the idea which I first proposed in 

 DieffenbacVs Journal of New Zealand, in 1843, that the plug of 

 the Anomia is only a modification of the byssus of the other 

 Acephala, especially of the lamellar byssus of the Arca^ has lately 

 been proved by the examination of the animal, which shows that 

 what has been considered as the muscles of the plug is really 

 the foot of the genus. Mr. Forbes has more lately proposed the 

 same theory, and compared the plug of the Anomia to the fibrous 

 byssus of the Pecten. 



If this is true of the Anomia, may not the tendon of the Tere- 

 bt^atula be a modification of the byssus of the Conchifera, and 

 the foot of that animal in its least developed state ? Dr. Loven 

 does not allow of this homologue, for he describes the Brachio- 

 podes as without any byssus. Dr. Loven considers the append- 

 ages of the Brachiopoda which have been called the foot, like 

 the arms of the Cephalopods, the papilla of the genus Clio, and 

 the lips of the Acephala, to be modifications of the veil or velum ; 

 probably because they are all placed near or round the mouth. 



Secondly, Professor Loven observes, " The Cephalopodes, Pte- 

 ropodes, Brachiopodes, and Pectinea have no pallial siphon ; the 

 Lamellibranchia Dimya have the branchial and anal often distinct; 

 the Gasteropoda Prosobranchiata have the branchial often distinct, 

 and the excretorial one rarely indicated, as in Pleurotoma and 

 Mangelia." To this I would remark, that in all the gill-breathing 

 mollusca I have observed alive, let them be Gasteropoda or Ce- 

 phalopoda, they have all had as distinct an in-going and out- 

 coming curi'cnt as the bivalve mollusca, generally situated in a 



