XIII] HELIX 291 



ordinarily long, attaining a length two or three times greater than 

 that of the body. In the Cuttle-fish the radula is present and the 

 jaw is developed into upper and lower beaks, like those of a parrot, 

 with which the animal tears its prey to pieces. The bivalves have 

 lost all trace of both jaws and radula : they live on the microscopic 

 organisms brought to them in the currents of water which they 

 produce, and so they do not need to masticate their food. 



The radula sac and the muscles and cartilages belonging to the 

 radula, form a swelling which is called the buccal mass. Behind 

 this comes the oesophagus or gullet, which appears narrow by 

 comparison, but its cavity is really as large as the space inside the 

 buccal mass. The gullet soon widens out into the first stomach or 

 crop, which is used for storing the food. On the outside or surface 

 of this two branching whitish structures are seen, the salivary 

 glands. They secrete a juice which runs forwards through two 

 tubes, the salivary ducts, opening into the buccal mass. The 

 saliva mingles with the food as it is being masticated. The crop is 

 situated in the hinder part of the neck, and behind it the ali- 

 mentary canal passes under the mantle-cavity and up into the 

 visceral hump. The great mass of this hump is occupied by a 

 brownish looking organ, called the liver. This, like the similarly 

 named organ in the Arthropoda, is a great mass of tubes lined by 

 cells of a deep brown colour : the tubes join together and event- 

 ually open by two main tubes, one above and one below, into a 

 dilatation of the alimentary canal. This swelling is the true 

 digestive stomach. It is probable that the " liver" assists di- 

 gestion by preparing a fluid which is poured into the stomach : 

 its function is thus not the same as that of the human liver. In 

 fact it must be confessed that the name liver has been recklessly 

 given by the older naturalists to any brown-coloured organ found 

 near the stomach of an Invertebrate. The part of the alimentary 

 canal behind the true stomach is called the intestine. It takes 

 a turn in the liver substance and then runs out of the visceral 

 hump along the right side of the body to open by the anus, which, 

 as we have seen, is placed just behind the respiratory opening. 



The central nervous system resembles that of the Annelida in 

 being made up of ganglia, each of which might be compared to a 

 miniature brain, connected together by means of commissures, that 

 is, bands of nerve fibres. The two largest ganglia, which are placed 

 above the oesophagus one at each side and connected by a com- 

 missure, are called the supra-oesophageal or cerebral ganglia, 



192 



