ORGANS OF ATTACHMENT. 7 



sometimes the organs of attachment consist of hooks and claws, which 

 serve to penetrate the underlying tissue or to lay hold of various 

 prominences. In Taenia solium (Fig. 4) and other tape-worms these 

 hooks have their basal end sunk within the tissues of the parasite, or 

 else, as in lice (Fig. 2) and the majority of the parasitic Arthropoda, 

 they are situated upon the extremities of the limbs. The various 

 bristles and other prolongations of the outer skin, so commonly met 

 with, may be safely included in the category of organs of attachment. 

 These, by contact with neighbouring parts of the body, not only in- 

 crease the power of resistance of. the parasite, but also prevent it from 

 being displaced in this or that direction, according to their disposition. 

 By the possession of setse of this kind, the male Bistomum (Bilharzia) 

 hcBmatobium is able not only to retain its position in the vena cava of 

 man, but also occasionally to advance against the blood stream into the 

 venous plexuses of the urinary bladder and rectum, so as to enable the 

 female, which it drags along with it, to lay its eggs in a convenient place. 

 Frequently several kinds of organs of attachment are found upon 

 the same parasite; an instance of this is Tcenia solium, which has 



Fig. 2. — Pediculus (Pktiarius) pubis. 



Fig. 3. — Trichocephcdus dispar, in situ. 



just been mentioned. Besides the hooks which are arranged in a 

 circle upon the summit of the head (Fig. 4), there are found a number 

 of suckers, which, together with the hooks, enable it to attach itself so 

 firmly that it is very difficult to remove it from its place. Comparing 

 the four suckers and their position upon the head with the single 

 terminal sucker of the leech and the two suckers of Bistomum (Fig. 

 1), we see that the organs of attachment in the parasites offer 

 quite as great differerO^'^^cdfcgiiMiffifasgafitent as in their structure. 



