WILLIAMSON, ON SHELLS OF CRUSTACEA. 45 



would be very desirable, were it possible, to examine each 

 of these species immediately after it had cast its shell, 

 tracing the various changes the new integument undergoes 

 in its progress to maturity. Unfortunately, inland students 

 have not the facilities for such inquiries enjoyed by their 

 more fortunate fellow-labourers who dwell on the sea-coast ; 

 but it occurred to me that much of the requisite information 

 might be obtained from the hermit crab. As is well known, 

 the claws of this species are as thoroughly calcified as those 

 of any other Crustacean. The anterior extremity of the 

 carapace is equally so; the'posterior portion of the latter 

 slightly, if at all j whilst the abdominal segments are as 

 devoid of calcific matter as any crab that has just cast its 

 shell. We are thus enabled to ascertain, without difficulty, 

 the relations between the superficial layers and the subjacent 

 derm or endoderm of Huxley. The careful study of this 

 species lias satisfied me that a well-marked basement mem- 

 brane interposes at all stages of development, between the 

 cellular endoderm or derm and the tubulated layers, whether 

 calcified or uncalcified; and that consequently the cells of 

 the endoderm cannot enter histologically into the outer tegu- 

 mentary layers forming the crustacean shell. 



The true derm or endoderm of Huxley, in the hermit 

 lobster, is thick and full of nuclei, colourless cells, and red, 

 blue, and brown pigment-cells. A section of the entire 

 integument of the soft carapace is represented in fig. 17, 

 where g indicates the derm, h its cells and nuclei, i its 

 pigment-cells, k the basement membrane, b the corium and 

 areolar layers blended, and a the pellicular layer. The pig- 

 ment-cells are sometimes stellate, at others appear as 

 rounded granular bodies. Fibrous threads often interlace 

 amongst the cellular elements. The basement membrane is 

 a thin tissue of the most delicate character. The appearance 

 represented on looking vertically through this membrane at 

 the subjacent derm is delineated in fig. 18, where the letters 

 indicate the same parts as in fig. 17. The uppermost 

 cells of the derm (18 h) are larger, more turgid, and more 

 regularly distributed in the deeper parts of the tissue, remind- 

 ing us of the aspect of cartilage-cells in growing bone 

 near the line of ossification. Immediately above the base- 

 ment membrane is seen a layer of membrane (fig. 17 b) 

 with a granulated texture. In the wholly uncalcified parts 

 of the integument this membrane is scarcely visible ; but it 

 becomes more so as we approach the head, where it pre- 

 sents a faintly areolated structure, like that seen in the 

 shrimp, whilst numerous patches occur in which the areola- 



