PROGRESS OF MICROSCOPICAL SCIENCE. 83 



microscope all caution novices against being deceived by it ; yet he 

 has good reason for thinking that many who are experts have been so 

 deceived. For a continued exhibition of the phenomenon, he believes 

 it to be essential that the matter should be so near the specific gravity 

 of the fluid as to remain in suspension. In the case of chalk and of 

 clay, the particles in time settle in contact with the glass, and then are 

 motionless until dislodged by jarring them. 



Structure of Crustacean Shells. — Mr. Edward Parfitt has communi- 

 cated to the Devonshire Association a paper on the above subject, 

 which is extremely valuable, and ought really to have been pub- 

 lished in this Journal. The subject, which is too large to admit 

 of om* giving a fair abstract, is in great measure a summary of the 

 views of other writers, although there are many perfectly original 

 matters mentioned. This author says that he has not seen it noticed 

 by any writer that the pigment layer (he speaks of it now as if it was 

 but one, instead of being made up of a series of layers) is still supplied 

 with pigmentum after the shell appears to be complete, but such 

 seems to be the case, for he was fortunate enough, when removing the 

 inside membrane, or chitonous layer, to observe the pigment vessels 

 in situ. This inner membrane is wrinkled into folds on the side next 

 tiie shell, and these folds are collected round an areolated centre, 

 having a larger orifice in the centre of the areolae. This is one of 

 the openings or communications between the animal and the corium ; 

 around these centres and on the folds are placed fit'om four to seven 

 pigment vessels, some of which were full and others empty. The full 

 ones were distended and filled with minute points of red colom"ing 

 matter. The vessels are composed of two membranes, an outer and 

 an inner ; the inner one does not entirely fill the outer one, as a con- 

 sideral)le space occurs between the two. These vessels have necks 

 sufficiently long to penetrate through the shell, and reach the pig- 

 ment layer. He could not discover the secretory glands or cells that 

 might be attached to those vessels, even with the highest magnifying 

 power at his command. The empty vessels somewhat collapse after 

 the pigment is discharged, as he observed several of them quite empty. 

 The author also describes in Cancer pagurus, on the upper surface of 

 the inner membrane, or on the last layer that has been added to the 

 shell, if a thin horizontal section be made, and a strong light be forced 

 through it, a number of peciiliar stellate bodies. The rays are set 

 round about a convex disk perforated with one or two holes. These 

 rays have somewhat the appearance of crystalline masses, and are 

 principally composed of calcareous matter secreted by the crab. The 

 centres of these rays correspond with, and are the representatives of 

 the little bosses on the sm-face of the shell ; and through the orifices in 

 the centre, appear to be the chief if not the only means for the 

 deposit of the earthy matter. It is these convex disks which give the 

 cui'vature to the various membranes as they are secreted and fixed ; in 

 fact the membranes are moulded over these, and they consequently 

 take their form. After a time these rays touch each other, but, so far 

 as he can see, they never overlap), but regular lines of demarcation are 

 formed ; and, curious enough, these lines form hexagons, in most 



