THE ANATOMY OF PENTASTOMUM TEEETIUSCULUM. 5 



into definite lines. Such spots are absent on the posterior part 

 of the annulus. 



The rounded anterior or head end of the body is not marked 

 by annuli. and its length is about five times the breadth of an 

 annulus. 



Mouth. — In the mid-ventral line a little way in front of the 

 first annulus is a distinct papilla, somewhat triangular in shape 

 with the angles rounded off, the whole projecting slightly from 

 the surface. The papilla arises from a depression, and is 

 surrounded by a narrow groove, which is also roughly triangular 

 in general form. The apex of the triangle formed by the 

 groove points forwards : the posterior side is at right angles 

 to the long axis of the body (figs. 2, 5), and forms a slit leading 

 directly into the mouth ; in fact, seen from the ventral surface 

 the mouth has simply the appearance of a slight swelling out 

 of the groove. The papilla corresponds to the structure to 

 which Hoyle gave the name of '^oral papilla " in P. protelis, 

 which Lohrmann also described as the " Mundpapille,'' but 

 which Stiles in P. proboscideum states is to be regarded not 

 as an oral papilla, but as an '^ Oberlippe.^' Whilst there is no 

 doubt that it performs the function of an upper lip, it is equally 

 certain that it has the form of a very definite papilla ; in fact, 

 *^oral papilla'^ most aptly describes its appearance and structure. 

 In P. tseniodes such a structure is apparently absent, the 

 mouth being described by Leuckart as a wide and gaping orifice 

 of oval shape. 



Hooks. — On either side of the mouth is placed a pair of 

 hooks. These are prominent structures, with sharp curved 

 protruding points, and bases sunken in special pits on the head 

 surface. The hooks serve for the attachment of the parasite, 

 and each, so far as the external anatomy of the animal is con- 

 cerned, consists of a strongly curved pointed portion and a 

 broader basal part. The depression in which the latter is 

 placed is bounded by a rounded elevated ridge of the external 

 cuticle ; within this, upon the base of the hook, lies a second 

 one, which is prominently marked where it passes across the 

 anterior end of the depression and hook. 



