296 MOLLUSCA " [CH. 



a separation of the sexes and the albumen gland is absent. The 

 penis in these. Mollusca is not a sac which can be turned inside out, 

 but a projecting lobe of the body, often of great size. In the most 

 primitive Mollusca the Soleriogastres the genital organ remains 

 throughout life a thickening of the wall of the pericardium or 

 coelom ; the eggs and spermatozoa drop into the pericardium and 

 find their way out by coelomiducts, just as is the case with 

 Annelida. 



This is the case also in Cephalopods, where, however, there were 

 originally four kidneys, and the one or two which serve as generative 

 ducts are specialised for this purpose ; thus the duct is in the male 

 prolonged into a papilla which serves as the penis. A commoner 

 case is for the generative organ to be closely connected with one 

 kidney and to burst directly into it. This is found in the simpler 

 Prosobranchiata, such as the Limpet (Patella), the Ear-shell 

 (Raliotis) and their allies. In Nucula and the simplest bivalves 

 there are two generative organs and they open into both kidneys ; 

 in the Pond-mussels (Anodonta and Unio\ and all more modified 

 forms, they open independently close to the kidney openings. There 

 is little doubt that in all Mollusca the tube conveying away the 

 generative products was originally a kidney or a part of one. 



Having got some idea of the arrangement of the organs of the 

 snail we must proceed to consider certain points about 

 of bofy * ne f rm f tne body considered as a whole. If we 

 except the genital opening, the head and neck of the 

 snail are exactly bilaterally symmetrical in their outer form ; on each 

 side there is a taste-tentacle and an eye-tentacle and the mouth and 

 the opening of the mucous gland are exactly in the middle line. Most 

 of the ordinary animals we see birds, quadrupeds, fishes, insects, 

 worms, etc. are bilaterally symmetrical with regard to external 

 appearance and many with regard to internal organs also. The 

 peculiarity of the snail is that, while it follows the ordinary rule as 

 far as the head, neck and foot are concerned, it departs from it 

 with respect to the visceral hump and the included organs. The 

 shell is, as we all know, spiral, but this shape is due to the shape of 

 the visceral hump contained within it, by the activity of the skin of 

 which the shell is produced. This spiral shape again is simply due 

 to one side being longer than the other, and it is connected with 

 the shortness of the right side that we find the opening of the anus 

 on the right side. In all bilaterally symmetrical animals this 

 opening is situated in the middle line, but in some of the marine 



