56 R. HOLMAN PECK, 



henoath these cilia is the blurred image of the lateral epi- 

 thelium (le.). 



The reader is referred to the figures and the appended 

 explanation of the plate for further detail. 



The narrow transverse diameter of the Mytilus gill-fila- 

 ment is TTo^t^ inch, the broad diameter (that from its outer 

 to its inner border) is T^rir^h inch. The interval between 

 neighbouring filaments varies according to pressure, but 

 averages -pjVuth inch. The large latero- frontal epithelial 

 cells which carry the long cilia are YTV^th inch in breadth 

 at the surface of origin of the cilia. 



Anodon. — Though Dreissena, on account of its position in 

 the same family with Mytilus, would natui"ally next claim 

 our attention, it will be best to take Anodon before passing 

 to Dreissena, since Anodon and Unio have been studied by 

 Dr. Posner, and give us a standard of gill-structure to which, 

 when referred, Dreissena is found to approach in important 

 respects, though retaining some features of the simpler 

 Mytilus-gill. 



In Plates V and VI a series of sections in different direc- 

 tions across the gill-plates of Anodon, and various enlarged 

 drawings of parts of the gill-plate, are reproduced. The ex- 

 planation which accompanies those plates will be found with 

 them to supply nearly all the information which I have to 

 give on the subject. 



In Anodon the sub-filamentar outgrowths are large and 

 abundant, so as completely to mask the primary tubular 

 character of the gill-filaments. In figs. 1, 2, 3, 4, dia- 

 grammatic views are given of transverse sections cutting the 

 gill-plate at right angles to the long axis of the filaments. 

 The sub-filamentar excrescences are seen to form a layer of 

 " lacunar " tissue, freely open to blood-turgescence and pro- 

 vided with large vascular trunks^ (true vessels with definitely 

 limited walls) which run parallel with the filaments, though 

 few and far between, as compared with these latter, which 

 I hold to be the primary vascular tubes of the Lamelli- 

 branch's gill. The large vessels occur at intervals of seven 

 filaments in the outer gill-plate, at intervals of twenty or 

 more in the inner gill-plate. They give rise to branches 

 running at right angles to them, forming part of the sub-fila- 

 mentar layer of each lamella. The branches are disposed 

 at definite intervals, so as to form with the filaments a regular 

 rectangular basket-work. The vertical element of the basket- 

 w^ork furnished by the filaments is perfectly regular and 



^ According to Kollman the silver treatment, brings into view here also 

 cell-oullines as iu the vessels of Vertebrata. 



