200 THE SCAPHOPODA 



1. 1.1. 1.1 (Fig. 1, C). The central tooth is simple and subquad- 

 rangular; the laterals stout and subtrigonal, tricuspidate, with 

 wide bases and reflected borders ; the marginals have the form of 

 subquadrangular non-denticulate transverse plates. 



The oesophagus is rather short and is provided with two 

 large lateral symmetrical pouches, directed ventralwards ; these 

 correspond to the oesophageal pouches of the 

 Polyplacophora and Aspidobranchia. 



The stomach is nothing more than the 

 most posterior bend of the digestive canal 

 (Fig. 182). It presents a small posterior 

 ciliated pyloric caecum, and receives the ducts 

 of the liver. The last-named organ is situated 

 behind the stomach and the rest of the alimen- 

 tary canal. It is made up of radiating caeca 

 (Fig. 182, li), gathered into two lobes which 

 extend into the sides of the mantle, and 

 open right and left into the so-called stomach 

 or posterior bend of the alimentary canal. 

 Though the two liver lobes are symmetrical in 



SipJionodentalium, an- , ii ,.. , , *. 



tenor end of body, dorsal the Dentalndae, they are no longer so in 

 manHe; I nrralT a r a I na S I s I ; Siphonodentalium. In this genus the principal 

 iv, right tentacular lobe, mass o f the liver lies in front of the gonad 



without the captacula ; V, , . ... 



buccai pouch ; vi, mouth ; and is continued posteriorly into two long 

 vm> t J parallel caeca extending to the extremity of 



the body ; the organ has no longer an apparent 

 symmetry, but all its radiating caeca are directed to the left side 

 and open into the " stomach " by a single orifice. 



The intestine bends forward (Fig. 182, in) and forms several 

 loops, all of which lie in the anterior part of the body, near the 

 buccai mass, and finally opens in the mid-ventral line behind the 

 visceral commissure. In the Dentaliidae there is an anal gland 

 on the right side of the rectum (Fig. 182, a). 



2. Circulatory Apparatus. The structure of the circulatory system 

 is exceedingly simple. There are no differentiated vessels, not even 

 a ventricle with well-developed muscular walls. At the most there 

 is a more contractile portion of the blood-system in the neighbour- 

 hood of the anus, but it has no afferent or efferent vessels, and is 

 continuous with the rest of the blood -spaces. These latter are 

 sinuses, without an endothelial lining, distributed between the organs 

 in the different parts of the body. The principal sinuses are : the 

 perianal, the pedal, the visceral, and the pallial. It is in the last- 

 named that the anterior dorsal and ventral portions are more clearly 

 defined and have something of the appearance of vessels. Two 

 buttonhole-shaped orifices, situated near the renal apertures, place 

 the perianal sinus in communication with the external medium and 



