ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 199 



pigmented ; there are noticeable differences between the epithelia of the 

 right and left margins. The sensory cells are very delicate, and there 

 are two kinds of them. Some are like the typical forms described by 

 Flemming, and have a capitulum which is only very rarely distinctly 

 marked off, and which is covered by stiff hairs ; the long neck has a 

 basal spindle-shaped swelling which contains the nucleus ; the nerve- 

 fibre which enters the cell is not in direct contact with the nucleus. 

 The second kind of sensory cell is represented by delicate structures 

 having nearly the form of an equilateral triangle ; the base carries the 

 cilia, while the tip is continued into a varicose terminal nerve-fibre. 



The left mantle margin is beset with five rows of tentacles ; those of 

 all but the fourth row are simple, and those of the fourth are branched ; 

 they are all pigmented, but the pigment is distributed irregularly; each 

 of these rows are separately considered, as are those of the right side. 

 While the musculature of the true mantle is exclusively longitudinal, 

 the tentacles have longitudinal, transverse, and circular muscles ; all 

 these kinds have numerous small nuclei, which stain intensely. 



The connective substance of the two halves of mantle which contain 

 the already mentioned swellings has a peculiar structure. In its meshes 

 there is a mass which, like that seen in the tentacles, does not appear to 

 be connected with definite histological elements. It is in some parts so 

 well developed as to completely obscure the other constituents of the 

 mantle-substance ; mucous cells are found in the amorphous mass. It 

 is possible that we have here to do with mucous cells, the substance of 

 which has passed into an amorphous mass. 



In the Ostreidae the circular nerve of the mantle lies closer to the 

 outer than the inner surface, and has a ganglionic cortex and nerve- 

 fibres ; it is surrounded by a delicate covering, which accompanies the 

 branches that pass into and occupy the axis of the tentacles. There 

 are two kinds of goblet-cells in the connective substance, and they are 

 present in large numbers. Their secretion appears to be protective. 

 The author is of opinion that Eyder's observations quoted by Sharp do 

 not serve to prove that Oysters are sensitive to light ; they are quite 

 blind, and their pigment-cells are indifferent structures. 



In the EadulidsB, of which Lima Mans and L. infiata are taken as the 

 types, Dr. Eawitz has already reported the presence of two forms of 

 marginal tentacles, which may be distinguished as sensory and glandular 

 filaments ; the differences in the microscopic structure of these are fully 

 described. There are three kinds of unicellular glands, all of which 

 agree in wanting a true cell-membrane. The secretion of the glandu- 

 lar filaments is, no doubt, of use in forming the nest of Lima, which 

 consists of various inorganic particles and remains of organic structures 

 held together by very delicate filaments. The great quantity of secre- 

 tion suggests that it has also some other function, and it may be that it 

 is a kind of defensive apparatus. 



In the Pectinidse and Spondylidse the edge of the mantle has 

 an extraordinary number of tentacles which are arranged in several 

 rows ; these vary in various species ; the innermost tentacles are the 

 longest. Pigment is found associated with indifferent epithelial cells, 

 and is so intensely developed in Spondylus as to render the investi- 

 gation of the minute structure exceedingly difficult. The several species 

 examined are discussed in great detail. 



In conclusion, the author gives an account of his examination of the 



