56 bulletin: museum of compaeative 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. The Branchial Organs are paired and situated one near the root of 

 each gill (Plate 9, Fig. 1, B, B) a little further back, and also a little deeper 

 in the mantle-cavity than the actual attachment of the gill itself. Tliey are 

 of a flattened ovoid form with the longer a.\is 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 (•?. I. c.) of connective tissue varjnng from 

 0.035 to 0.015 mm. in thickness. 



2. Beneath this is a delicate membrane (s. m.) which stains deeply and in 

 wliich 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. Tiie 

 nuclei are more numerous and the cell boundaries less distinct in the layer 

 wliich 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. 



Towarils 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. f.). In its deeper portion is an ovoid mass of 

 material similar t<j that composing the greater part of the central ma.ss, but 

 not so clearly marked off into cells (<7.). 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 tis-sue lies below the central 

 mass (c. t. 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 boumlary of the organ. 



6. A thin layer of pigmented cells covers the posterior half of the organ on 



