42 TAPEWORMS. 



usually most destructive where insanitary conditions exist. 

 "Water plays a considerable part in the distribution of these pests, 

 a fact which will apply to the majority of parasitic worms. A 

 typical tapeworm consists of a permanent head, known as the 

 scolex (fig. 10, a), and a long segmented body, each segment 

 being known as a proglottis (b). Between the scolex and the 

 body there comes a constricted area, the neck (fig. 12, n). This 

 is the growing area of the worm, the area from which fresh 

 segments are constantly being budded off. The proglottides 

 increase in size towards the tail end. Each proglottis is prac- 

 tically a perfect animal in itself, as each has a separate set of 

 sexual organs. When the segments are ripe — that is, sexually 

 mature and full of ova — they fall off : sometimes they disunite 

 separately, at others in groups of four or five, and pass out to 

 the exterior in the animal's excrement. 



The liead or scolex (fig. 10, a). — This so-called head has no 

 mouth ; there are usually two or four suckers (.s) upon it, 

 enabling it to become attached with the additional aid of two 

 rows of hooks (li) to the mucous membrane of the host's in- 

 testines. Hooks may be absent. In some species the scolex is 

 drawn out in front into a kind of snout, the rostelluia, and then 

 hooks are present surrounding its tip. 



"We find four chief modifications of scolices ; (i) In the 

 Tcenice, in which there are four suckers and two sets of hooks 

 on the rostellum ; (ii) Bothrioceplialus, in which there are two 

 suckers ; (iii) Acanthobothrium, which are provided with a 

 complicated set of suckers and beset with numerous hooks ; and 

 (iv) Tetrarhynchus, which has four protrusible probosces, also 

 beset with recurved hooks. 



The neck (fig. 12, n). — Here note the first traces of segmenta- 

 tion. At first faint transverse stripes are seen, not extendin<T 

 right across the neck ; farther down they pass from side to side, 

 and small segments are thus marked off', becomincr larger and 

 more distinct the farther we get from the scolex. The neck 

 is narrow and constricted. 



