STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 29 



often broken or blunted. The posterior part of the lingual rib- 

 bon usually has its margins rolled together, and united, forming 

 a tube, which is presumed to open gradually. The new teeth 

 are developed from behind forwards, and are brought successively 

 into use, as in the sharks and rays amongst fishes. In the 

 hulladce the raclm of the tongue is unarmed, and the business of 

 communicating the food is transfen'ed to an organ which re- 

 sembles the gizzard of a fowl, and is often paved with calcarious 

 plates, so large and strong as to crush the small shell-fish 

 which are swallowed entire. In the 

 aplysia, Avhich is a vegetable-feeder, 

 the gizzard is armed with numerous 

 small plates and spines. The stomach 

 of some bivalves contains an instru- 

 ment called the » crystalline stylet,'' ^.^ ^^ Gizzard of Bulla * 

 which is conjectured to have a si- 

 milar use. In the cephalopods there is a crop in which the 

 food may accumulate, as well as a gizzard for its trituration. 



The liver is always large in the moUusca (fig. 10) ; its se- 

 cretion is derived from arterial blood, and is poured either into 

 the stomach, or the commencement of the intestine. In the 

 nudibranchs, whose stomachs are often remarkably branched, 

 the liver accompanies all the gastric ramifications, and even 

 enters the respiratory papiUee on the backs of the eolids. The 

 existence of a 7'enal organ has been ascertained in most classes ; 

 in the bivalves it was detected by the presence of mic acid. The 

 intestine is more convoluted in the herbivorous than in the car- 

 nivorous tribes : in the bivalves and in haliotls it passes through 

 the ventricle of the heart ; its termination is always near the 

 respiratory aperture (or excurrent orifice, when there are 



* Fig. 17. Gizzard of bulla lignaria (original). Front and side view of a 

 half-grown specimen, with the part nearest the head of the animal down- 

 wards ; in the front view the plates are in contact. The cardiac orifice is in 

 the centre, in front ; the pyloric orifice is on the posterior dorsal side, near 

 the small transverse plate. 



