ANATOMY AND PHYSIOLOGY OF PATELLA VULGATA. 613 



muscles, are described and figured by Geddes {Trans. Zool. Soc, x. 485). Lan- 

 kester (Ann. and Mag. Nat. Hist., 1867) confirms Cuvier's discovery of a crop 

 of salivary glands, and says they (the salivary glands) open into the buccal 

 cavity by four ducts. Dall (Amer. Jour. Conch., 1871) figures and describes 

 the radula; as does also Gray (Sys. Disti-ib. of Mollusca in Brit. Mus.). 



3. Circulatory System. — The circulatory system consists of a branchial vein 

 and veinlets, a heart, and two efferent vessels. 



The branchial vein is easily seen in fresh specimen as a large clear belt 

 beneath (ventral) the circle of gill lamellae, when examined in section. It 

 cannot be distinguished from a large lacuna. No special lining of epithelium 

 is visible; its walls are composed of connective tissue. Crossing it at regular 

 intervals can be seen clear veinlets, which however do not open into the 

 branchial vein. These veinlets spring from the mantle skirt beyond the bran- 

 chial vein, — i.e., nearer its edge, — and open into lacunas at the origin of the 

 gill lamellae. From the gills again arise veinlets (PI. CLI. fig. 38, e), which 

 open into the branchial vein (PI. CL. fig. 28). The branchial vein sur- 

 rounds the entire mantle skirt, and the ends unite to form one vessel, which 

 passes over the left shoulder to enter the pericardium at its extreme left 

 corner. The vessel, as it passes -round the pillar-like termination of the 

 circular muscle, becomes surrounded by fibres of muscular tissue, which are 

 continuous with the fibres of the auricle into which the vessel immediately 

 opens (PI. CLI. figs. 41 and 43). 



The pericardium, which is continuous with the visceral integument, but 

 thicker, is composed of very tough connective tissue, to which muscle fibres 

 are attached on its inner aspect for the support of the heart. It is lined 

 internally with squamous epithelium, often scarcely visible owing to its thinness. 



The heart consists of two chambers, an auricle and a ventricle. The 

 auricle is large, and very thin-walled. It is attached to the pericardium at 

 some points, especially in front. The attachment at that point is extensive, 

 and the writer has not been able to convince himself that no communication 

 exists between the auricle and the very vascular cephalic portion of the mantle. 

 He has not been able to force any injection into the auricle from the lacunae, 

 nor has any injection passed into the lacunae from the heart, so far as he can 

 make out. It is possible, however, that further injection experiments, which he 

 purposes trying, may lead to different results. [These experiments the author 

 has since made, with the result that he feels convinced that the mantle in the 

 head region acts as an accessory respiratory organ, and that the blood from 

 that area enters the auricle by openings in the " attachment " referred to.] 

 The auricle is very distendible. It opens into the ventricle, which lies be- 

 neath it, by a slit which is guarded by muscle fibres rather curiously placed. 

 The fibres are continuous with others which form a network inside the 



