EXPLA.NA.TION OP PLATE Ml—conlimied. 



show the trabec'ulje of the lacunar tissue aiul their nuclei. This and fig. 

 20 are drawn to a larger scale than any of the other sections. 



Fig. 19. — Section of a fragment of subfilamentar lacunar tissue, to 

 show the nature of the wall of one of the large vertical vessels. A small 

 horizontal trunk is seen connected with it. Compare fig. 11. 



Fig. 20. — Diagram of a gill-filament seen from the supra-lamellar 

 surface, and cut so as to show (1) the surface epithelium, (2) the higher 

 lateral epithelium, and (3) the deeper lateral epithelium. Tlie diagram 

 is constructed from optical sections (from a drawing by Mr. Lankester). 



Fig. 21. — The large cells of the latero- frontal epithelium; three as 

 seen from the supra-lamellar surface ; one in the plane at right angles to 

 this. 



Fig. 22. — Epithelium ft-om the gill-filament. 



Fig. 23. — Epithelium of the supra-lamellar surface of a gill-filament ; 

 intended to show the relative size, shape, and position of the cells as seen 

 in a transverse section. This drawing must be taken as correcting de- 

 ficiencies in figs. 9, 10, 11. 



Dreissena. 



Fig. 24. — Portions of two transverse sections of a gill-plate of Dreis- 

 sena, showing the various arrangement of the large vertical channels or 

 vessels in relation to the interlamellar junctions. 



Figs. 25— 29.— See below, Plate VII. 



Fig. 30. — Dense form of lacunar tissue, with closely packed fusiform 

 cells. This tissue is found in the interlamellar junctions of Dreissena, 

 and also in those of Anodon, but it seems periodically and locally to give 

 place to the looser typical form of lacunar tissue with trabeculse. 



Fig. 31, — Similar dense lacunar tissue, forming a branched comple.v 

 trabecula. 



Plate VII. — Dreissena. 



Fig. 25. — Transverse section through a gill-plate rff Dreissena, show- 

 ing two interlamellar junctions, one of which (the left) consists at this 

 point of dense lacunar tissue with fusiform cells; whilst the other has 

 trabeculse only and floating corpuscles. The upper lamella exhibits five 

 filaments in section ; the section passing along the horizon of a transverse 

 fibrous junction. The lower lamella has its six filaments free from 

 transverse junction. Note the small size in these of the subfilamentar 

 outgrowths as compared with Anodon. Obj. 8, oc. 4. 



Fig. 26. — Two filaments in transverse section more highly magnified. 

 The peculiar outgrowth of the chitinous axis of the tilament, and its 

 relation to the lacunar tissue, is to be noted. The section is not far from 

 the horizon of a transverse interfilamentar junction. By the side of the 

 filaiiients is a view Irom the supra-lamellar aspect of one of the large 

 laterij-frontal cells. (From a drawing by Mr. Lankester.) 



Fig. 27. — View from the supra-lamellar aspect of the surface of a 

 portion of three gill-filaments of Dreissena, showing the few nuclei of 

 the frontal epithelium and the bordering latero-frontal epithelium. 



Fig. 28. — Diagram showing part of three filaments, transverse 

 fibrous junctions, subfilamentar tissue, and stomata of water- passages of 

 Dreissena. 



Fig. 29. — Diagram of part of four filaments, with two transverse 

 fibrous junctions. 



N.B. — In none of the plates are the figures drawn to a constant scale. 

 For the actual dimensions of the objects figured the reader is 

 referred to the text. 



// 



