ENTOZOA 



437 



species the canal system of the lemnisci opens into that of the introvert in front of the 

 cuticular ring, and is thus completely independent of that of the trunk. If we assume 

 that the head of my species corresponds with the introvert of other forms which have 

 lost its introvert sheath, the lemnisci open into the same region of the skin as they do 

 in other Acanthocephala. 



The nuclei of the subcuticle and of the lemnisci are very remarkable ; they corre- 

 spond in structure with those described by Hamann in Neorhynclms clavaeceps, in which 

 species according to this observer both the skin and the lemnisci retain in the adult 

 their embryonic condition. As in Neorhynclms the number of nuclei is very small, some 

 twelve to twenty seem to suffice for the whole of the subcuticle, and perhaps two to four 

 for each lemniscus. The structure of the nucleus shows a most striking- resemblance to 

 an amoeba with rather short pseudopodia (figs. i6, 20, and 23). No single nucleolus 

 can be detected, but numerous chromatin particles are present, and in some a distinct 

 vacuole can be observed. These nuclei are scattered about in a most irregular fashion ; 

 not one may be seen in a number of consecutive sections, and then perhaps three or 

 four may appear, and from their large size persist through several sections. The nuclei 

 lie, as a rule, embedded in the substance of the subcuticle ; more rarely they are found 

 in the lacunae. Although there is no proof, one is tempted to believe that the nuclei 

 wander through the subcuticle and lemnisci in an amoeboid manner, and that the small 

 number of nuclei which are found in these tissues is compensated for partly by the large 

 size of each, but more especially by their mobility. Similar amoeboid nuclei undoubtedly 

 move about, fuse with one another, and undergo fission in the subcuticle of the larval 

 forms of Neorhyuchus clavaeceps. 



Within the subcuticle and completing the skin on the inner side, is a layer of 

 circular muscles, and still more internally a layer of longitudinal muscles (figs. 16 and 

 25). The muscles of these layers are but a single fibre thick, and they are not very 

 uniformly present. The circular layer is most complete in the region of the trunk, and 

 I have figured a section to show this (fig. 22). The longitudinal layer is even less 

 definite, but scattered fibres can be detected here and there (figs. 16 and 25). Each 

 fibre appears to be spindle-shaped, and in the circular muscles has the striated portion 

 only on its outer face, forming a thin band ; the inner half of the fibre consists of vacuo- 

 lated strands of protoplasm in which is a nucleus. The longitudinal layer of muscles 

 alone is continued over the lemnisci (figs. 19 and 24). These muscles are not covered 

 on their inner side by any layer of epithelial cells, neither does any such layer cover the 

 ligament, but both tissues lie freely exposed to the fluid of the body-cavity. 



In the more typical Acanthocephala the anterior end of the body terminates in a 

 hollow eversible portion provided with rows of hooks whose number and shape have a 

 certain systematic value. This introvert can be withdrawn, not into the general body- 

 cavity, but into the cavity of the introvert sheath, which is shut off from the general 

 body-cavity by a double (Echinorhynchidae) or a single (Neorhynchidae) wall. The 



