THE SCAPHOPODA 201 



admit of the expulsion of blood during violent contractions of the 

 body (Fig. 182, o). There is no specialised respiratory apparatus. 

 Respiration is effected by the internal surface of the mantle, 

 particularly by the anterior ventral region. 



3. Excretory Organs. The Scaphopoda have two symmetrical 

 kidneys, situated in front of the gonad on the ventral side of the 

 middle of the body (Figs. 181, q, and 182, k). They have the form 

 of two short but fairly wide sacs with pleated walls, lying between 

 the intestinal mass and the stomach. They have no communica- 

 tion with one another and have no reno-pericardial duct. They 

 open to the exterior on either side of the anus. 



4. Nervous System and Sense Organs. The nervous system of the 

 Scaphopoda comprises the same four pairs of principal nerve ganglia 

 as are found in the Gastropoda and Lamellibranchia, in addition to 

 the stomato-gastric system. The cerebral ganglia are joined to one 

 another and are situated on the dorsal side of the oesophagus : they 

 innervate the proboscis with its palps and the tentacular lobes and 

 captacula. Each cerebral ganglion is in close juxtaposition to the 

 corresponding pleural ganglion (Fig. 182, c.g, pl.g), which innervates 

 the mantle. The cerebral and pleural ganglia are united to the 

 pedal ganglion of the same side by a long connective which is 

 apparently single in the distal part of its course, but bifurcates just 

 before it reaches the cerebral and pleural ganglia, sending a branch 

 to each. The two pedal ganglia are situated in the foot (Fig. 

 182, p.g) and are attached to one another. 



The visceral commissure takes its origin from the pleural ganglia. 

 It is rather long, and bears on the posterior part of its course two 

 symmetrical visceral centres (Fig. 182, v.g) in the form of simple and 

 ill-defined ganglionic swellings, lying on either side of the anus close 

 beneath the tegumentary epithelium, and thus resembling the 

 visceral ganglia of the Lamellibranchia. These two centres are 

 united by a commissure passing in front of the rectum. 



From the cerebral ganglia there arises as in many other 

 Molluscs an infra-oesophageal labial commissure (Fig. 182, la.c), 

 bearing a ganglion on either side, from which a branch of the stomato- 

 gastric commissure properly so called (Fig. 182, st.g) is given off, as 

 is the case in the Polyplacophora, Aspidobranchia, and Cephalopoda. 

 The stomato-gastric commissure passes to the ventral side of the 

 oesophagus, between it and the buccal bulb, and bears two or four 

 symmetrical ganglia on the middle of its course. The labial com- 

 missure also gives off a nerve on each side, which passes to the 

 subradular organ and terminates below it in a ganglion. 



The Scaphopoda have only three differentiated sensory organs : 

 the captacula or tentacular filaments, the subradular organ, and the 

 otocysts. The tentacles, which are seemingly tactile and olfactory 

 organs, are dorsal in position and have the form of flattened lobes 



