GENERAL CHARACTERS OF THE MOLLUSCA. 



68 I 



which may or may not be provided with an apparatus of teeth, or 

 may be furnished with tactile processes. The digestive tube is com- 

 monly of considerable length, and usually shows a gullet, into which 

 salivary glands pour their secretion, a stomach, and an intestine. 

 The rectum terminates at the anal aperture (fig. 551, a), which is 

 primitively at the hinder end of the body, but which in many forms 

 ultimately becomes shifted further forward. The blood is colour- 

 less ; and a definite heart, consisting of a ventricle and one or two 



Fig. 551. — Diagram of a Mollusc (altered from Lankester). m, Mouth; a, Anus; /, Liver; 

 y, Foot ; sh, Shell, attached to the mantle ; h. Heart ; pe. Pericardium ; n, Kidney, opening in- 

 ternally into the pericardium, and externally near the anus (k); br. Gill ; eg. One of the cerebral 

 ganglia, connected by commissures with the pleural ganglion (j>g) and the pedal ganglion (Jg) ; 

 " isceral nerve-cord extending backward, and carrying visceral ganglia (zg). 



auricles, is always present, and always has the function of propell- 



ing the aerated blood through the body. 



Respiratory organs are not always developed, the function of 

 respiration being in some cases discharged by the thin walls of the 

 mantle-cavity. In the majority of the Molluscs, however, there are 

 definite respiratory organs, a portion of the mantle being specialised 

 for this purpose. In the Lamellibranchiata, and the branchiate Gas- 

 tropoda, the breathing-organs are in the form of lamellar or pectin- 

 ate gills ; and the same is the case with the Cephalopoda. In the 

 pulmonate Gastropoda, in which respiration is aerial, a pulmonary 

 sac or air-chamber is produced by the folding of a portion of the 

 mantle, over the interior of which the pulmonary vessels are dis- 

 tributed, and the external opening of which is placed on the side 

 of the neck. 



The nervous system consists of paired ganglia united by com- 



