56 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
consists of a number of delicate fibres arranged in bundles, passing outwards 
and forwards, parallel to each other. Its breadth is about 0.24 mm, and its 
thickness 0.12 mm. 
III. Tug BRANCHIAL ORGANS are paired and situated one near the root of 
each gill (Plate 9, Fig. 1, B, B) «little further back, and. also a little deeper 
in the mantle-cavity than the actual attachment of the gill itself. They are 
of a flattened ovoid form with the longer axis directed rather forwards and 
inwards. The anterior and more superficial portion has a pale yellowish and 
rather lustrous appearance, behind which is a crescentic band, which looks as 
though it were due to subcutaneous pigment. The greatest length is about 
0.75 mm, and the breadth somewhat less. 
The structure of these branchial organs (Fig. 9) is much simpler than that 
of either of the sets just described. The following parts may be distinguished 
proceeding from the surface downwards: — 
1. Over the surface is a thin layer (s. l c.) of connective tissue varying from 
0.005 to 0.015 mm. in thickness. 
2. Beneath this is a delicate membrane (s. m.) which stains deeply and in 
which rounded nuclei can be made out here and there. It has an almost uni- 
form thickness of about 0.006 mm. 
3. The central mass (c. m.) constitutes by far the greatest part of the whole 
organ. It consists of a parenchymatous tissue of cells (Fig. 11), the largest 
of which are about, 0.04 mm. in length, the smallest being less than half as 
much. The cell boundaries are very distinctly marked, and the protoplasm 
stains but slightly with haematoxylin. The nuclei are spheroidal and stain 
very deeply, so deeply that in most cases the nucleolus cannot be seen. The 
nuclei are more numerous and the cell boundaries less distinct in the layer 
which immediately underlies the delicate membrane just described. 
Here and there may be seen amongst the cells a sort of lacuna in which is 
a granular mass with one or two nuclei, Sometimes this mass fills the lacuna, 
in others it seems to have shrunk away, in which case threads of protoplasm 
may be seen stretching out from it across the intervening space. 
Towards the posterior edge the structure of the central mass undergoes a 
change (Fig. 10) ; instead of a parenchyma of rather elongated cells it assumes 
the appearance in its more superficial part of a fibro-cellular mass with nuclei 
scattered irregularly in it (s.c.). In its deeper portion is an ovoid mass of 
material similar to that composing the greater part of the central mass, but 
not so clearly marked off into cells (el.). There is a kind of spiral striation 
obscurely indicated, as though the structure had in some way been formed 
round a centre. 
4. A rather thick layer of very close compact tissue lies below the central 
mass (c. f. c). It stains very deeply, and hence its minute structure is very 
difficult to decipher, but it apparently consists of a granular substance in which 
no definite elements can be made out. In the anterior half this layer forms 
the boundary of the organ. 
5. Athin layer of pigmented cells covers the posterior half of the organ on 
