61 G R. J. HARVEY GIBSON ON THE 



• The under surface of the mantle differs in many respects from the upper 

 surface, and resembles the structure of the gills so markedly that no doubt it is 

 to be looked upon as functionally a respiratory organ, just as the gill lamellae 

 are to be considered as morphologically processes of the mantle. The epithelium 

 covering the under surface of the mantle and the lamellae is also columnar; 

 at the tip it is regular and low ; over the thicker part of the mantle the cells 

 are very small and crushed, and the columnar structure is not always evident. 

 Their outlines on surface view are ragged, and their free ends are embedded in a 

 homogeneous membrane. Their inner ends are also ragged, the processes being- 

 sunk in the subepithelial connective tissue (PI. CL. fig. 26). 



Where the mantle again thins, and where the gills rise, the cells become 

 regular columnar, as on the dorsal surface, and spring from a muscular layer 

 similar to but thicker than the most superficial layer mentioned, as underlying 

 the epithelium of the dorsal surface. The fibres are, however, frequently oblique. 

 Above the origin of the gill lamellae the epithelium is also columnar, and is 

 covered by a very distinct cuticle. The muscle in this part of the mantle is 

 arranged in a thin radial layer separated by connective tissue from the epithelium 

 (PI. CL. fig. 27). 



The body of the mantle in its upper half is divided by transverse muscular 

 bands into a series of quadrilaterai'compartments, which are lacunar blood spaces. 



The thicker ventral or outer part of the mantle is composed in the main of 

 connective tissue and muscle with scattered nerves, forming a dense network 

 in which, however, there are lacunae — one large one, which is the branchial vein, 

 and several of smaller size. The whole mantle is well supplied with nerves. 

 There are usually five or six branches, which run at uniform distances round the 

 mantle skirt. Each is enclosed in a connective tissue sheath, and each divides 

 up into fibres and fibrillae, which are distributed to the muscular fibres. 



Occasionally a specimen is found in which the branchial vein is enormously 

 swollen. In such cases, the tissue of which the mantle is composed can be 

 studied to much better advantage (PI. CL. fig. 28). The connective tissue 

 corpuscles are specially well developed (PI. CL. fig. 29). 



The free part of the mantle skirt over the head is similar in structure to 

 that just described. The membranous portion has a structure like that of the 

 visceral integument. It is composed of three layers — a superficial pigmented 

 layer composed of cubical cells, a middle layer of connective tissue and a few 

 muscle fibres, and a deep layer of cubical cells similar to those of the visceral 

 integument (q.v.). The middle layer contains many lacunar blood spaces 

 (PL CL. fig. 20). 



Each gill lamella is composed of two flattened plates of connective tissue, 

 with a few muscular fibres connected by transverse bands of connective tissue 

 (PI. CL. fig. 30). A single layer of columnar epithelium covers over each 



