556 THE INVERTEBRATA 



round hole on the right side which is the aperture of the mantle cavity 

 or pneumostome. In the marine gasteropods the mantle cavity has a 

 wide opening to the exterior, though a part of the mantle border 

 (siphon) is modified to form a special channel by which fresh water for 

 breathing may be drawn in by the action of the cilia clothing the gill. 

 But in the air-breathing pulmonates where the cavity is converted 

 into a lung, the injury of delicate respiratory tissues by evaporation 

 must be avoided, and a pumping mechanism for renewal of air 

 established. The restriction of the respiratory aperture is one of the 

 necessary modifications. If a section is drawn across the lung of a 

 snail it will be seen that the mantle forms the roof of the cavity and 

 is covered with ridges in which run pulmonary veins converging to- 

 wards the auricle. The floor of the cavity is arched and has a layer of 

 muscles, which contract rhythmically. When they contract, the arch 

 flattens and air is drawn in and at the limit of contraction a valve slides 

 across the pneumostome. When the muscles relax, the cavity de- 

 creases in size and exchange of gases with the blood in the roof vessels 

 is facilitated by the increase of pressure of the contained air. Then the 

 pneumostome opens and air is expelled ; the subsequent contraction 

 of the floor muscles brings in a fresh supply. This "breathing " is not 

 so regular or so frequent as in a vertebrate ; moreover, it may cease 

 altogether in the winter when the snail hibernates. 



In dissection, a cut is made underneath the collar and another 

 under the rectum and the roof of the mantle cavity turned back 

 so as to show the pericardium enclosing the ventricle and single 

 auricle, and the kidney, which is a yellow organ consisting of a 

 number of folds covered by cells containing uric acid. The ureter is 

 a thin-walled tube which runs along the right border of the mantle 

 cavity parallel to the rectum and opens just behind the pneumo- 

 stome and above the anus. Here again is a diff'erence from the marine 

 gasteropods in which the anus and kidney aperture discharge inside 

 the mantle cavity, faeces and urine being swept away by the respira- 

 tory current. The pericardium and the kidney represent the coelom 

 in the snail and, as is usual in Mollusca, their common derivation is 

 shown by the connection of the cavities by the renopericardial canal. 

 The coelom, though thus represented, does not constitute the peri- 

 visceral cavity. On cutting the floor of the mantle cavity and con- 

 tinuing the cut forward towards the mouth a large body cavity is 

 revealed which contains the anterior part of the alimentary canal and 

 the greater part of the reproductive organs. This is a haemocoele 

 almost as well developed as that of arthropods. Its connection with 

 the rest of the blood system and the general course of the circulation 

 may be briefly described here as follows : the ventricle pumps arterial 

 blood through a single aorta which soon divides into an anterior aorta 



