TYPE MOLLUSC A. 305 



branchia which lies in front of the heart, whence the name of 

 the order, but in a few genera the original left gill also per- 

 sists. In many forms a gland is developed in the floor of the 

 mantle-cavity close to the rectum — hence called the adrectal 

 gland — which in some forms, e.g. Murex secretes a purple 

 pigment. The rotation of the mantle-cavity and the associated 

 organs has called forth a crossing of the pleuro-parietal nerve- 

 cords, a feature which is lacking in the other orders and 

 therefore forms a characteristic of the Prosobranchs. 



In all but a few cases the members of the order are bisex- 

 ual, the unpaired reproductive gland lying in the visceral 

 hump. The oviduct has in connection with it one or more 

 receptacula seminis and dilates into a glandular uterus in 

 which the eggs are supplied with the albumen in which they 

 are usually imbedded and also surrounded by a shell. In 

 the males, except in the more primitive forms, there is present 

 a well-developed intromittent organ or penis (Fig. 133, ^e), 

 situated upon the right side of the head or neck and there- 

 fore removed at some distance from the opening of the vas 

 deferens into the mantle-cavity. A groove or tube extends, 

 however, from the reproductive orifice to the grooved or tubu- 

 lar penis, and along this groove or tube, by the ciliary action 

 of the cells lining it, the seminal fluid is carried. 



1. Suborder Diotocardia. 



This suborder includes the more primitive Prosobranchs, 

 in which, although a considerable rotation has occurred, yet 

 nevertheless the abortion of the organs of the original left 

 side of the body has not been carried very far. Thus, except 

 in Patella and some allied forms, there are two auricles to the 

 heart, although in Turbo, Trochus, Neritina, and allied genera 

 that of the right side (i.e., the original left one) does not com- 

 municate with the ventricle. Attention may again be called 

 to the fact that in those forms which possess two functional 

 auricles the ventricle and pericardium have wrapped them- 

 selves round the rectum which seems to perforate the ventri- 

 cle. Such forms as Haliotis, Fissurella, and Pleurofomaria pos- 

 sess two branchiae, but in the majority of the members of the 



