74 COMPARATIVE ANATOMY 



donta, and both have the same mndamental structure as the 

 nephridium of the earthworm, consisting of a coiled tube, 

 divisible into a glandular and non-glandular part, and opening 

 by one end into the ccelom (pericardial cavity in molluscs), 

 and by the other end to the exterior. But close as the resem- 

 blance is, it is probable that the two organs are not homo- 

 logous, for the excretory organs of molluscs appear to arise as 

 outgrowths of the ccelom, whereas the nephridia of chaetopods 

 are ingrowths from the external epiblast which meet and acquire 

 openings into the ccelom. Hence we have refrained from call- 

 ing the former organs nephridia. It should be noticed that 

 there is only one excretory organ in the snail, and that it lies 

 to the left of the rectum. If the visceral hump were rotated in 

 the direction of the hands of a watch so as to bring the anus 

 back into a median and posterior position the excretory organ 

 would lie on its right. It is, in fact, the primitive right organ, 

 its left-hand fellow having disappeared. 



A large blood-vessel runs in the wall of the mantle-cavity 

 parallel to and not far distant from the rectum. It is formed 

 by the union of numerous branches which ramify in the mantle, 

 being particularly numerous in its right side. The main vessel 

 is known as the pulmonary vein. It receives a considerable 

 branch from the excretory organ, and runs into the pericardium, 

 expanding within it to form the thin walled auricle. The 

 auricle is succeeded by a pear-shaped muscular ventricle, 

 from which an artery passes backwards into the visceral mass. 

 Immediately on entering the visceral mass the artery divides 

 into two branches, one of which ascends the spire and supplies 

 the visceral hump with blood. The other branch turns for- 

 ward and supplies the head and foot. The details of this 

 circulatory system may be studied in Marshall and Hurst's 

 " Practical Zoology," and will not be described here, but it is 

 important to notice that the branches of the arteries open into 

 a system of large and irregular lacunar spaces, from which the 

 blood is afterwards collected by definite venous trunks, and 

 carried to the pulmonary chamber. The anterior part of the 

 gut and the generative organs lie in a large space of this kind, 

 which might easily be mistaken for ccelom. It is not ccelom, 

 however, but a greatly enlarged blood space which has en- 

 croached upon and taken the place of ccelom, the latter being 

 represented only by the small pericardial cavity and the cavity 



