614 R. J. HARVEY GIBSON ON THE 



ventricle, rendering it more or less of a sponge. From the arrangement of 

 the muscle fibres round the auriculo-ventricular opening, contraction of the 

 ventricle must bring about contraction of these muscle fibres, and cause 

 occlusion of the opening, thus preventing regurgitation (PI. CLI. fig. 44). 



The ventricle is practically a sponge of muscle fibres. It is oval, and 

 more or less pointed in shape at the ends, where it opens into two aorta?. 

 The long axis of the ventricle runs transversely, — that is, from right to left. 

 The left aorta, or efferent vessel, passes into and supplies the circular muscle. 

 The right, after passing out of the pericardium at the posterior right-hand 

 corner, breaks up and opens into a very perfect system of lacunae. Although 

 the writer has made many injections with carmine gelatine in fresh specimens, 

 he has never been able to trace the vessels beyond a very short distance. 

 The right efferent vessel runs beneath the rectum. 



The wall of the auricle is composed of diagonally arranged belts of 

 muscle' which are held together by single nucleated muscle and connective 

 tissue cells. The fibres seem embedded or covered by a connecting mem- 

 brane with small nuclei scattered in it, which may represent squamous 

 epithelium (PI. CLI. fig. 39). The ventricle has a similar structure, but 

 the muscle fibres cross and recross in the interior of the ventricle. 



The muscle fibre of both auricle and ventricle is composed — the auricle 

 partly, the ventricle wholly — of a species of striped fibre common enough in 

 invertebrate hearts. The transverse striation is not very distinct, and it gives 

 the individual fibres rather a granular appearance. Bundles of fibres are 

 enclosed or wrapped round by nucleated connective tissue cells (PL CLI. figs. 

 39 and 45. 



The blood corpuscles (PI. CLI. fig. 42) are colourless and amoeboid. 

 They appear something like spiny balls; though here and there flatter, more 

 irregular corpuscles are visible. Each contains one or more nuclei, and 

 is composed of granular protoplasm. When a coagulum is formed, the pseudo- 

 podial processes anastomose, and the clot under the microscope resembles 

 a plasmodium of Monobia. 



With the exception of Cuvier's (loc. cit.) brief remarks on, and rather 

 meagre drawing of, the heart, and Williams' reference to the structure of the 

 blood corpuscles (quoted by Dall in his paper on " Limpets " in the Armr. 

 Jour, of Conchology, 1871), the writer has been unable to find any observation 

 of importance on the circulatory system. Lankestek (art. " Mollusca," Ency. 

 Brit.. 9th ed.) makes a brief reference to it, but does not enter into any detail. 



4. Purificatory System. — (a) Respiratory System. — The functional gills are in 

 the form of lamella;, arranged round the inside of the skirt of the mantle. They 

 are morphologically processes of the mantle. They are attached to the mantle 

 in an oblique manner, so that a transverse (i.e., vertical) section may cut through 



