20 COMPARATIVE ANATOMY. 



In Chdyosoma, shell tesselated; Chevreulius, bivalved. Bol* 

 tenia has calcareous granules at pedicle. Siliceous particles 

 in integument of Salpa. 



Lamellibranchiata. Skeleton bivalved (shell), placed right 

 and left to body ; opened by elastic ligament at hinge. The 

 line of hinge may be toothed or smooth. 



Gasteropoda. Skeleton generally univalved (shell). Oper- 

 culum, a horny or calcareous disk attached to foot and em- 

 ployed in opening and closing mouth of shell. It is supposed 

 by some to represent the second valve of the Lamelli- 

 branchiata. Shell generally whorled from right to left, 

 rarely from left to right as in Physa; rudimentary in Limax 

 (slug), multivalvular in Chiton. 



Pteropoda. Skeleton (shell) univalved; papery, as in 

 Hyalca and Cleodora. 



Cephalopoda. Skeleton of two kinds, external and in- 

 ternal. External, representing the shell of other molluscs, 

 is univalved, and may be either an elaborate chambered 

 shell (Nautilus), or rudimentary. When of the latter kind, it 

 is developed from the dorsum of mantle, and is made up 

 within a closed sac of a number of corneous pen-like bodies, 

 as in Loligo (squid), or a single, light, calcareous piece, as in 

 Sepia (cuttle-fish). Such skeletons are used as floats. Internal 

 form is cartilaginous. It is confined to four segments sur- 

 rounding the brain, named as follows: brachial, cephalic, 

 dorsal, and articular. This is the first appearance of an in- 

 ternal skeleton in the animal kingdom. 



Structure of skeleton of molluscs is chiefly that of columns 

 of calcareous matter arranged transversely to the shell. 

 Great variety exhibited in the detail of arrangement of these 

 columns. Inner layer porcellaneous; outer layer epidermic; 

 nacreous layer (mother-of-pearl), when present, in the inner 

 aspect of porcellaneous layer. Pearls are formed by a depo- 

 sition of the secretion of the nacreous layer upon some ex- 

 traneous body lying between the mantle and the shell. This 

 may be a grain of sand or the body of a parasite. 



Tubular structure of nacreous table in Anomia and Area; 

 to the perforate type in Brachiopoda, few exceptions known. 



