ENTOZOA 



435 



of these parasites was divided into three regions, — a head, a collar, and a trunk ; and, in 

 fact, they have an almost ludicrous resemblance to a young Balanoglossus with one or 

 two gill-slits (figs. II, 12, and 13). On investigating their anatomy it at once became 

 evident that the animals belonged to the group Acanthocephala, and, further, that they 

 differed from the other members of the group in the absence of what is perhaps their 

 most characteristic organ, — from which, indeed, they take their name — the hooked 

 proboscis or introvert. Careful inspection failed to reveal any trace of a scar or mark 

 where the introvert might have been broken off; and although in the absence of hooks 

 and introvert sheath, &c., the anterior part of the body which I have called the head is 

 as unlike the typical introvert as possible, still in its relation to the lemnisci and to the 

 ligament it occupies the position of that organ, and until we can get further information 

 I think the best plan is to regard this part of the body as equivalent to the eversible 

 part of more normal forms. 



The second of the three regions into which the body is externally divided is shorter 

 than the head and smaller in diameter ; it may be termed the collar. The third or 

 posterior region, which may be called the trunk, is the longest and the most slender of 

 the three ; behind it tapers to a point where the orifice of the genital duct is situated, 

 and this end of the animal is always a little turned up (figs. 11, 12, 13, 17 and 23). The 

 exterior of the collar and trunk are smooth or lightly wrinkled, but the head is covered 

 with a number of small depressions or pits which give it a very characteristic appear- 

 ance, and which are well seen in sections. The head is attached to the collar by a 

 narrow neck, which is surrounded and concealed by the edge of the collar. This is 

 obvious in sections (figs. 15 and 23). All the specimens were somewhat shrivelled and 

 apparently distorted. The largest measured 3 '5 mm. in length, the smallest 2-5 mm. ; 

 had they been fully distended they would probably have been i to i '5 mm. longer. 

 The body-cavity of the head is continuous with that of the neck, and the latter opens 

 freely into the cavity of the trunk (fig. 23). The first-named space is by far the largest. 

 The lumen of the collar region is reduced by the great thickness of the walls of this 

 part of the body, and both here and in the trunk much of the internal space is occupied 

 by the lemnisci and the reproductive organs. 



The skin is one of the most characteristic features of the Acanthocephala, and as 

 far as I know is only paralleled by that of the Nematodes, but it possesses certain 

 features not found in the last-named group. The whole body is covered by a thin 

 cuticle which does not vary much in thickness in the different regions of the body, and 

 which is invaginated a short distance into the genital pore. Beneath this is the true 

 epidermis, or subcuticle as it is called ; this has in my specimens the usual structure met 

 with in the group so well described by Hamann, and consists of a matrix of a fibrillar 

 nature, the fibrils being as a rule arranged radially, in which are embedded a certain 

 number of amoeboid nuclei (figs. 16 and 20). This tissue is much thicker in the region 

 of the collar than elsewhere, and it is thicker in the trunk than in the head. It is pierced 



