80 MR W. TURNER AND DR H. S. WILSON ON THE 



abdomen. They lie side by side, and almost parallel to each other. Each ai*m 

 ends superiorly in an expanded structm'e, concave on its upper aspect, which 

 performs the office of a clasper. The adjacent surfaces of the two claspers are 

 flat, and, when the arms are in their normal position, these surfaces are accu- 

 rately adapted to, but not continuous with each other, so that they can be drawn 

 asunder without any difficulty. The arms, therefore, cannot be regarded as 

 blended together at their extremities. Lying in the concave upper surfaces of 

 the apposed claspers is a curved cartilaginous-like bar (fig. 2, a), the convex outer 

 surface of which is closely embraced by them. The bar has in its interior a dis- 

 tinct cavity. Projecting from the surface of the bar which is in apposition with 

 the claspers, is a small papilla, along the axis of which the central cavity of the 

 bar extends. Between the superior margins of the flattened apposed surfaces of 

 the claspers a slight depression exists, into which the papilla of the bar fits. Two 

 small oval openings exist at the bottom of this depression, one belonging to each 

 clasper. Each communicates with a long and slender tube, which passes through 

 the substance of the clasper to be continuous with the central canal of the cylin- 

 drical part of the arm. The substance of the bar, when examined microsco- 

 pically, without the addition of any reagent, appears to be structureless. After 

 digesting thin slices in ether, and subsequently boiling them for some time in 

 acetic acid, a very beautiful cellular structure is perceived. These cells are about 

 the size of primordial cartilage cells. In them the nucleus is elongated, and be- 

 tween it and the cell-wall a delicate concentric arrangement is seen. There is 

 evidently a difference between the chemical composition of the bar and the 

 chitinic integument of the animal, for on steeping a specimen for some hours in 

 a chromic acid solution, whilst the colour of the latter was very slightly affected, 

 that of the bar was changed to a dark brown, almost black. 



Both the claspers and the cylindrical portions of the arms contain powerful 

 muscles, by the contraction of which the arms can be very much shortened, at 

 which times they have a very crenulated appearance. These muscles are trans- 

 versely striped, and are arranged both in longitudinal and circular bundles. A 

 canal extends along the centre of each arm, which communicates at its root with 

 the general cavity of the body. This canal is irregularly subdivided by connective 

 tissue bands passing across it. 



It is by means of the arms that this crustacean attaches itself to the skate on 

 which it is parasitic. When examined in situ, the arms could be traced passing 

 along the space which separates two of the nasal laminae from each other. They 

 then enter a canal of calibre sufficient merely to contain them. This canal 

 dilates at its end into a comparatively large space, lined by a distinct smooth 

 vascular membrane. In this cavity the claspers, with the bar which they 

 embrace, are situated. The bar lies transversely, its long axis corresponding to 

 that of the space in which it is situated ; and as the canal, in which the cylindri- 





