THE ANATOMY OF A PTEROPOT). 81 



is formed, lying on the posterior aspect of the body, and opening to 

 the exterior by a slit at the ventral end, i.e. at the mouth of the 

 shell/ 1 ) 



There is no distinguishable head, and of head appendages only 

 two tiny, easily overlooked tentacles lying just behind the fins. 

 These have evidently suffered degeneration, showing in their minute 

 size, but little resemblance to the great organs so familiar upon jthe 

 head of the snail. 



Two slight eminences guard the entrance to the mouth. Within 

 this a radula or teeth-bearing ribbon is found, whence a long 

 oesophagus leads straight backwards or rather upwards, into a dilated 

 stomach. The intestine is continued backwards for some distance, 

 then abruptly turns and passes forwards (downwards) to open laterally 

 into the mantle cavity at a point on the left side. 



Lying close to the intestinal bend, is the anterior end of the 

 enormous sausage-shaped secretive organ which for convenience we 

 may term liver (I). It runs backwards parallel with the anterior 

 half of the ovo-testis. 



As regards muscular tissue, such is developed sparsely except in 

 the fins, and in a great strand of fibres that originates from a point 

 only a little below the apex of the shell, runs parallel with the 

 ovo-testis and liver, thence forward and to the right, to the oral 

 end of the body, and to the copulatory organs. Its name, the 

 retractor muscle, denotes its function. 



On the right side, in the region of the intestine, lies an elongated 

 cylindrical organ, the nephridium or kidney. This has at one 

 end an opening communicating with the exterior, while at the other 

 the end turned towards the apex of the visceral hump a passage 

 is found leading into the pericardium. Probably this is a means for 

 introducing sea-water into the blood at stated intervals, thus giving 

 an additional and interesting function to the nephridium. 



The heart, lying dorsal to the nephridium, and like it, on the 

 right side of the body, consists of a globular ventricle and of a 

 delicate auricle. From the former a large artery is given off, leading 

 into several smaller branches. These however, instead of in turn 

 leading into capillaries and thence into veins, open into an irregular 

 chain or network of indefinitely shaped spaces (lacunae) disposed 

 in the tissues, and without definite walls. From these the impure 

 blood is gathered into a large venous or pericardial sinus, whence it 

 is passed into the auricle. 



(1). To arrive at the right application of the terms dorsal, ventral, anterior, and 

 posterior, to the body of a Pteropod, one must picture it as in fig. G, PI. XIII, the 

 mouth downwards and the apex of the shell directed upwards. 



