ACEPHALA. 5 



Class III. Gastropoda or snails. The under side of the body 

 forms a muscular foot, suitable for gliding (PI. xviii, f. 16). 



Glass IV. Scaphopoda. Head rudimentary. Foot vermiform, 

 furnished with lobes. A degraded form of the encephala having 

 relationships with Vermes, one of the most obvious of which is 

 its shell, a hollow cjdinder open at both ends, the spiral shell of 

 the gastropod unrolled into a tube, reduced to the last degree 

 of simplicity in form. 



The shell, when present in the Encephala, is usually external 

 and spiral, as in the helix, the whelk, nautilus, etc., but there are 

 notable exceptions in all the classes : 



In the Cephalopoda a whole order (Octopoda) is mostly without 

 any shell, whilst another order (Decapoda) is mostly furnished 

 with a simple shell called a pen, developed beneath the dorsal 

 skin. Ver}^ few species of recent cephalopods have an external 

 spiral shell, but during the earlier geological periods thousands 

 of species of such animals existed. 



In the pteropods (sometimes considered an order of Gastro- 

 poda) the shell is glassy ; globular, conical, cylindrical or spiral, 

 but is often wanting. 



In the Grastropoda the shell is usually external and spiral, but 

 sometimes tubular or even reduced to a low cone, the point of 

 which is the beginning of its growth, the large end the mouth or 

 aperture. Sometimes the shell is internal ; occasionally internal 

 and rudimentary, consisting of an irregular calcareous plate or 

 several grains, and frequei^tly altogether absent. Portions of 

 this class are terrestrial or fluviatile, but most of its species are 

 marine : the other classes of encephala are strictly marine animals. 



The Chiton or coat-of-mail shells form an aberrant group, the 

 shell consisting of eight distinct calcareous plates, usually over- 

 lapping, and lodged in a coriaceous skin or mantle. 



In the Scaphopoda or Solenoconcha (dentalia or tooth shells) 

 the shell is external and tubular. 



All the Encephala are provided with a tongue or lingual ribbon, 

 the superior surface of which usually bears rows of teeth used 

 in rasping their food; many have also plates, called jaws, lodged 

 within the mouth, and aiding in comminuting food. The cepha- 

 lopods have external horny beaks, suitable for cutting or tearing. 

 None of these organs are found in the Acephala. 



II. ACEPHALA. 



This term is fi-equently used as synonymous with the single 

 class which it contains. 



Glass V. Pelecypoda, Lamellibranchiata or Gonchifera (PI. 

 iii). The bivalve mollusca are familiarly represented by the 

 oyster, clam, mussel, etc. The name Lamellibranchiata is more 



