418 THE ANATOMY OF INVERTEBRATED ANIMALS. 



into two lobes, though it may be slit or perforated where 

 it forms the wall of the branchial chamber (Haliotis, Fis- 



surella). 



Very generally, the prosoma bears tentacula and eyes ; 

 and a distinct head being thus recognizable, these Mollusks 

 have been named Cephalophora, in contradistinction to the 

 acephalous Lamellibranchs and Brachiopods. 



The mantle commonly gives rise to a shell, which may 

 either be a more or less calcified cuticular product of the epi- 

 dermis, covering the outer surface of the mantle, when it 

 constitutes an external shell, as in the Lamellibranchiata 

 and Brachiopoda ; or it may be developed within a sac in 

 the interior of the mantle, as an internal shell. In neither of 

 these cases is it ever a bivalve shell divided into two lateral 

 portions. 1 Usually it is in one piece (univalve), but in one 

 group, the Chitonidm, it consists of a number of pieces (not 

 exceeding eight), arranged in longitudinal series along the 

 middle line. 



Calcareous matter is very commonly diffused, in the form 

 of granules, through the connective tissue, and often takes 

 the form of spicula (e. g., Doris). 



The mesosoma is generally prolonged into a muscular foot, 

 which may be provided with lateral appendages, the epipodia. 

 And, on the haemal aspect of the posterior portion of the foot, 

 a chitinous or shelly plate, termed the operculum, may be 

 developed. This operculum appears to be the analogue, if 

 not the homologue, of the byssus of the Lamellibranchs, and 

 is certainly not homologous with either of the valves of the 

 shell of the latter, which are pallial structures. The edge of 

 the mantle forms a free fold which nearly or entirely sur- 

 rounds the mesosoma ; and in one genus, DentcUium, the 

 margins of the mantle unite for the greater part of their 

 length : in all the rest they remain free. A space is inclosed 

 between the lobes of the mantle and the mesosoma. Usually 

 this space is much larger on one face of the body, and con- 

 stitutes the pallial chamber. As a rule, the branchiae are 

 lodged in this chamber, and the anus opens into it. 



In a very few Odo?itophora, the symmetry of the body is 

 undisturbed; that is to say, the mouth and the anus are situ- 

 ated at opposite ends of the axis of the body, and the haemal 



1 The singular bivalve plates, termed Aptychus, which occur in the Ammo- 

 nitidce, whatever their nature may be, are obviously not homologous with the 

 shell of ordinary Mollusks, which is represented by the chambered shell of 

 the cephalopod. 



