MOLLUSCA. 319 



Iii most Mollusca the Cephdlophora as opposed to the Acephala 

 or Lamdlibr and data the anterior part of the body is marked off 

 as a distinct head, and bears the mouth, tentacles, and special 

 organs of sense. The dorsal part of the body behind the head 

 constitutes the main mass of the animal, and is called the visceral 

 sac. The integument of the visceral sac constitutes the mantle or 

 pallium, and is nearly always folded near the junction of the 

 visceral sac with the head and foot. This fold is the mantle-fold, 

 or pallial-fold, and the groove enclosed by it is the mantle- groove, 

 or pallial-groove. The mantle-groove contains the gills, anus, and 

 renal openings, and is nearly always deeper in the neighbourhood 

 of these structures than elsewhere, thus constituting the mantle- or 

 pallial-cavity proper. The outer surface of the mantle generally 

 secretes a calcareous shell though this structure may, by secondary 

 modification, be absent in the adult. The head and foot are 

 attached to the shell by paired and symmetrical muscles in the 

 Placophora, Scaphopoda, Lamellibranchiata (retractors of the foot), 

 and Cephalopoda (retractors of the head and funnel) ; and in Gastro- 

 poda by a single muscle the columellar or spindle-muscle. 



The central nervous system consists of a circum-oesophageal ring 

 with a uniform coating of nerve-cells, or the latter may be specially 

 aggregated into ganglia the cerebral or supra-oesophageal, and the 

 pedal or sub-oesophageal. In some forms the pedal or ventral 

 portion of this nerve-ring may extend as a pair of long cords into 

 the foot (Chiton, Haliotis, etc.). Sometimes there is a second circum- 

 oesophageal commissure connecting the cerebral and pedal, and pro- 

 vided with a ganglion the pleural. These various ganglia give 

 off nerves the cerebral to the head, cephalic sense organs and 

 lips, the pedal to the foot, and the pleural to the mantle and 



gills. 



In addition to this somatic system, as we may call it, there is 

 the visceral system, which generally consists of two commissures 

 with ganglia in their course. Of these there is (1) the stomato- 

 gastric commissure, which starts on each side from the cerebral, 

 and is completed ventrally to the oesophagus ; the ganglia upon 

 it are called the buccal, and supply the buccal mass and stomach ; 

 and (2) the visceral commissure, which starts on each side from 

 the pleural, and is completed posteriorly ventral to the intestine. 

 The ganglia upon this commissure vary in number in different 

 forms, and are called the visceral. They innervate the intestine, 

 vascular, renal, and reproductive organs. The stomatogastric and 



