200 THE SCAPHOPODA 
1.1.1.1.1 (Fig. 2, 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. 
if Though the two liver lobes are symmetrical in 
Siphonodentalium,  an- 35 ; 
terior end of body, dorsal the Dentaliidae, they are no longer so in 
vevile: Il eadulae mass, Siphonodentalium. In this genus the principal 
DR iat ade mass of the liver lies in front of the gonad 
buccal pouch; VI, mouth; and is continued posteriorly into two long 
lotic? VY? *™™l yarallel 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, im) and forms several 
loops, all of which lie in the anterior part of the body, near the 
buccal 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 
Fic. 183. 
