7^ ENTOZOA. 



of T. affinis as compared with tlie human species. According to 

 "Wilson "it is a slender shaft, 3^ of an inch in diameter, and 

 from two to three lines in length. It is situated along the ventral 

 border of the worm, occupying somewhat more than its posterior 

 third. It is enclosed, in the first instance, by a thick envelope or 

 sheath, and lies in contact with the internal surface of the integu- 

 ment ; and, nearer the tail, it enters the intestine and carries with 

 it a thin membranous sheath, which is prolonged to the anal open- 

 ing. The apex of the style is conical and sharp ; its base is broad 

 and gives aittachment to a strong muscular fasciculus, which arises 

 at some distance farther forward from the longitudinal muscular 

 fibres of the parietes of the animal, and performs the office of a 

 retractor muscle." By the base of the spicule, I understand 

 Wilson to refer to the upper or infundibuliform end, which I 

 have represented in the accompanying outline drawing from 

 nature (Fig. 16, i'). The membranous sheath itself is not in- 

 variably uniform in breadth, being often globular, flask- shaped, 

 or pyriform at its free extremity, where the spicule emerges. Two 

 of these varying conditions are represented in the annexed cuts 

 (Figs. 16 and 17), whilst the ordinary uniform state of the 

 sheath is shown in the Plate (Fig. 4). To myself these appear- 

 ances were at first very puzzling, but I afterwards found that these 

 varying conditions were simply due to the degree of protrusion at 

 which the membrane had arrived. Externally the membrane is 

 covered throughout with the characteristic minute, conical, sharply- 

 pointed spines, whose apices are directed backwards towards the 

 body of the animal. The lower part of the style is somewhat 

 curved or scimitar-shaped, but I have not discovered any groove 

 on its surface for conducting the semen, such as has been described 

 as existing on the spiculum of T. dispar. In one case, however, I 

 found the flask-shaped formation at the end of the sheath to con- 

 tain a mass of seminal particles (Fig. 17, /.;), and behind this 

 swelling the membrane was contracted at intervals so as to 

 form a striking series of regular annulations. 



