of the Cocoon of a Leech. 303 



face of the body of the cocoon and bounds the deeply sunken 

 areas of its interstices rises from its surface in the form of a sharp 

 edge, and as the free fibres are given off at the angles where the 

 imbedded fibres meet, they naturally at this point assume the form 

 of a three-winged fibre, and this form they maintain throughout 

 the whole of their length, as represented by PI. XVIII. fig. 8. 

 with a linear power of 94. 



Every one is familiar with the horny cases surrounding the 

 ova of certain fishes, and of the finely-spun horny threads with 

 which they are fixed to the stems of Gorgonias and other bodies ; 

 but in these cases the fibre is simple and cylindrical, as might 

 naturally be expected, while in the fibrous tissue of this singular 

 cocoon it is three-winged, and anastomoses as regularly and as 

 beautifully as the fibres of the horny sponges of commerce. How 

 the animal produces this beautiful and complex structure, is a 

 question which it will be exceedingly interesting hereafter to solve. 



The coriaceous substance of the body is of about the thickness 

 of a stout sheet of writing-paper, the centres of the areas being 

 much thinner than the other parts. When a section of one of its 

 thickest portions at right angles to its outer surface was examined 

 by transmitted light with a power of 94 linear, it appeared to be 

 composed of four or five layers of nearly equal thickness, as re- 

 presented by PI. XVIII. fig. 10. When the exterior surface was 

 examined under similar circumstances with a power of 1000 linear, 

 numerous cytoblastic vesicles were observed irregularly dispersed 

 over its surface, but without the appearance of nuclei ; but, on 

 the contrary, when the inner surface was thus examined, it was 

 seen to be nearly uniformly covered with well-defined nucleated 

 cytoblasts, the nuclei in many cases being angular, as represented 

 with a power of 1000 linear by PI. XVIII. fig. 9. From the la- 

 minated structure exhibited in figure 10, it is probable that the 

 production of tessellated cellular tissue is not continuous, but that 

 it occurs at intervals, and is produced by a series of efforts, in a 

 similar manner to that in which the successive layers of cartila- 

 ginous substance are produced by Helix aspersa when about to ex- 

 tend the lip of its shell in the spring of the year. But there is 

 an essential difference in the circumstances of the two cases. In 

 the shell the cytoblasts are developed and their peculiar office 

 performed while in contact with the living body whence they ema- 

 nate, while in the cocoon this cannot well be the case, as the ani- 

 mal immediately separates itself from it. Their presence and de- 

 velopment therefore appear to indicate that vitality to a certain 

 degree remains in the horny substance of the cocoon, and which 

 vitality may probably continue in action until the proper office of 

 the cocoon has been attained. Dr. Johnson, in treating of Hirudo 

 vulgaris, describes the singular mode of the production of the co- 



