THE BEANCHIOGASTEEOrODA. 37 



serves to protect the animal when retracted into its shell : but 

 as the operculura is developed from the foot and not from the 

 mantle, it can obviously have no homology with the valves of 

 either a Brachiopod or a Lamellibranch. The Branchiogastero- 

 poda (Fig. 15) commonly possess a distinct head, provided with 



Fij. 15. 



ig. 15. Section of a female^whelk (Buccinum}. The organs marked t and h are removed 

 from their proper places ; the others are seen in situ, a, mouth ; 6, gullet ; c, head ; d, 

 foot; e, operculum; /, free 'part of the mantle; g, that }rt which invests the 

 visceral mass lodged within the shell ; A, a gland of unknown function connected with 

 the gullet; i, crop; k, stomach ; I, intestine; m, rectum; n, heart; o, aperture of 

 the renal organ ; r, mucous gland developed from the wall of the mantle cavity ; s, 

 oviduct ; t, salivary gland. The arrows indicate ihe position of the branchiae. The 

 cerebral, pedal, and parieto-splanchnic ganglia closely surround the gullet, and the 

 latter send off a lor.g ganglionated cord towards the heait and branchiae. 



a pair of tentacles and a single pair of eyes, both of which are 

 supplied with nerves from the cerebral ganglia. Cephalic eyes 

 of this kind are not known in the LameUibrancJiiata, But the 

 characters which most definitely distinguish the Branchiog aster o- 

 poda are to be found in the alimentary canal. The cavity of 

 the mouth is invariably provided with an organ which is usually, 

 though not very properly, called the tongue, and which might 

 more appropriately be denominated the " odontophore." It 



