Introduction to Animal Morphology. 293 



These vessels end mostly in lacunary spaces, either separate 

 from (Patella) or communicating with the body cavity. In 

 the liver, foot, some parts of the mantle, velum, and the intra- 

 pericardiac wall of Aplysia, capillaries exist, which, in the last 

 named place, form a rete mirabile connected to the aorta. In 

 Haliotis the lacunae are specialized, and, though wall-less, 

 look like an intermediate form between lacunae and true capil- 

 laries. From these lacunae the blood returns to the heart 

 either by slits in the pericardium (Heteropoda, Pontolimax, 

 Actceonia, Elysia), or is collected in large spaces near the 

 auricle (Abranchia), or enters venous trunks that bring it to 

 the gills. There are no vessels in Rhodope nor Entoconcha, 

 and none but the aorta in Pontolimax and Phyllirhoe ; there 

 are no veins in Opisthobranchiata. In Prosobranchs, anterior 

 and posterior blood-sinuses, beginning by open ends, convey 

 the blood to the branchial artery and to the kidney, where 

 there is a close venous plexus, and whence comes a portal 

 vein to the branchial artery and to the auricle. 



There is a water-vascular system commencing by 

 one* or more large, or several f small openings in the 

 foot, with a network therein, and opening into the body 

 cavity. Water taken in by these makes the foot large 

 and prominent, while it mixes with the blood, dilutes 

 it, and perhaps aerates it as well. In Dolium there 

 is an opening into the vascular system beside the 

 anus. J The respiratory organs may be absent 

 (Abranchiata, as Rhodope), or the surface, with its 

 superficial lacunae, may be respiratory (Dermobran- 

 chiata, as Pterotrachea, Pontolimax, Phyllirhoe). In 

 others the gills are either parallel plates or cylin- 



* Pyrula, Strombus, Buccinum. 



t Natica canrena. 



X In Pleurobranchus there is a small opening behind the genital orifice 

 leading to the stem of the branchio-cardial vessel,^bet\veen the point at 

 which it receives a branch of the circum-dorsal sinus and the auricle. 



