38 MR JOHN RENNIE 



The colour is dead white. The rostrum, which was found fully extended in only a 

 few instances, is very nearly cylindrical, being rather thicker at the base than at the 

 tip. In one specimen its length was found to be 1'14 mm., its width at tip '13 mm., 

 and at the base '23 mm. It carries about 28 rows of hooks, of which there are 10 in a 

 row. These hooks differ slightly in form in different regions of the rostrum, but are 

 quite evidently modifications of one type (fig. 2). In the row nearest the tip they are 

 slender, and point more directly outward than do those near the base. Here they are 

 shorter and thicker. Actual measurements are- 

 Hooks of first row . . . . . . -114 mm. long. 



Hooks at base of rostrum ..... '07 mm. long. 



The rostrum at its base occupies the centre of a region of peculiar form. Here the 

 anterior part of the body is conical, and from the tip of the cone the rostrum emerges. 

 Around the cone at its base there is a shallow circular depression, the outer border of 

 which rises to form a thickened rim. This rim marks the widest part of the whole 

 worm, and defines the posterior border of the front body-region. The whole of this 

 region cone, depression, and rim is covered with minute, slightly recurved spines, 

 regularly arranged in rows. The average measurement of these spines is 0'7 mm. in 

 length. Behind this the body curves backward in a bowl-like manner, beyond which it 

 is bent sharply at a right angle, and continued in a much narrowed tail portion. The 

 free end. differs slightly in the two sexes, as will be described. This posterior body- 

 region bears spines of the same type as those occurring in the anterior part, but upon 

 a restricted area only. The shape of the parasite is such that, when attached by the 

 rostrum to the alimentary canal of the host, one side will be applied along its whole 

 length to the intestinal or stomach-wall. This side, together with a very short portion 

 encircling the tip, constitutes the spiny area ; the whole remaining surface of the 

 posterior region is quite naked. Since the object of the spines is primarily to assist in 

 maintaining secure fixation, the significance of their one-sided distribution in an animal 

 of this peculiar shape is apparent. Their occurrence around the tip (genital area) 

 suggests a possible use in copulation. 



External Sex Characters. 



There is well-marked sexual dimorphism. This is not uncommon in the Acantho- 

 cephala. According to de Marval " les Acanthocephales presentent un dimorphisme 

 sexuel souvent tres accentue", et qu'en these generale, les femelles sont beaucoup plus 

 grandes que le males, voire meme quelquefois geantes." The present species exhibits 

 the unusual peculiarity that the males are larger than the females. In support of this 

 interesting fact, which is an exception to what generally obtains amongst animals, it 

 seems worth while to quote the following measurements : 



(ROT. soc. OF EDIN. PROC., VOL. xxvi., PP. 438-439.) 



