522 MR. A. D. MICHAEL ON THE 



edge of one joint and are inserted into the posterior edge of the next more distal joint; 

 the exact jmrt of the edge from near which they respectively arise or into which they 

 are respectively inserted depending in each case upon the direction of the motion the 

 muscle is to give. 



The External Cuticle. 

 (Figs. 33, 34, 41, 42, 43.) 



This can hardly be said to form part of the internal anatomy, which is the subject of 

 this paper ; but as it is invaginated in several places, and is thus associated with the 

 inner organs, and is also somewhat peculiar and interesting in itself, I think it best to 

 say a few words regarding it. 



The skin of Bdella is extremely fine : it is composed of a thin epidermal layer which 

 in B. Basteri is only about 1 ^ thick ; this layer is highly flexible and quite transparent 

 and colourless ; the whole of the brilliant red pigment which makes the creatures so con- 

 spicuous lies in the deeper layers, I. e. hypoderm and fat layer, and on the exterior of 

 the internal organs themselves. The exterior of this epiderm (fig. 33) is marked with 

 the fine, wavy, jmraliel lines so common in many of the soft-bodied Acarinn, e. g. the 

 Sarcoptida) and others ; in B. Basteri they are about 600 to the millimetre. These lines in 

 Bdella are of an exceptional character ; instead of being mere markings they are ridges, 

 which are thicker at the base where they start from the cuticle than at their distal 

 edges, so that a section through one of tliem is like the section of a narrow knife-blade ; 

 this appearance is increased by the marvellous sharpness and fineness of the edge. The 

 fine lines springing from the exterior of the cuticle in all drawings of sections of the 

 whole creature (figs. 34, 41, 42, 43) are not hairs, but are these ridges cut through. The 

 height of the ridges is considerably more than tlie thickness of the solid part of the 

 epidermal layer : in B. Basteri tlie ridges in their liighest part are about 2"5 /x liigh, 

 more than twice the thickness of the layer ; but the height of the ridge, /. e. the 

 amount of its projection, is not equal in all parts, for the following reason : — Each 

 ridge is not an even one of a fixed thickness and projection throughont, it is a series 

 of small drawn-out swellings of different sizes and lengths joined by their fine ends ; the 

 ridge never ceases and never loses its distal knife-edge, but it swells out and contracts 

 irregularly, so that when looked at from above it appears like a string of irregular beads, 

 each bead drawn out to a point at both ends and joined to the next bead by an extremely 

 fine thread. The height of the ridge in section naturally depends on whether the section 

 cuts a large or a small swelling or falls between two swellings. It is not possible to 

 depict this irregularity in drawings on the scale of the figures above enumerated. 



The hypoderm is a much thicker tunic than the epiderm, and is of the usual character 

 in the Acarina, with loosely joined living cells of irregular size which have very distinct 

 elongated nuclei and elliptical nucleoli. 



