CESTODA. 327 



the head armature is very weak, and consists of a lobed fringed 

 expansion. The apex of the head often ends in a conical projection, 

 the rostellum, which is armed with a double circle of hooks, while 

 the lateral surfaces of the head are furnished with four suckers 

 (Ttenia, fig. 263). In other cases only two suckers are present 

 (Bothriocephalus) ; or we find suckers of more complicated structure 

 and beset with hooks (Acomthoibothrium), or four protrusible probosces 

 beset with recurved hooks (Tetrarhynchus); while in other genera 

 the head armature presents various special forms. . 



That portion of the animal which follows the hea.d and is dis- 

 tinguished as the neck shows, as a rule, the first traces of com- 

 mencing segmentation. The rings, which are at first faintly marked 

 and very narrow, become more and more distinct and gradually 

 larger the further they are removed from the head. At the pos- 

 terior extremity the segments or pro- 

 glottides are largest, and have the 

 power of becoming detached. After 

 separation they live independently 

 for a long time, and sometimes even 

 in the same medium. 



The simplicity of the internal or- 

 ganization corresponds with the simple 

 appearance of the external structure. 

 Be'neath the delicate external cuticle 

 is a matrix consisting of small cells, 



, , , . -.-, FIG. 263. Head of Tcenia . soJium, viewed 



in which are scattered glandular ceils. from the front (apical sur face), with 

 Beneath the matrix there is a delicate rosteiium and double circle of hooks. 



The four suckers are visible. 



superficial layer of longitudinal mus- 

 cular fibres, and next a parenchyma of connective tissue, in which 

 strongly-developed bundles of longitudinal muscular fibres, as well as 

 an inner layer of circular muscles, are embedded ; both these muscular 

 layers are traversed, principally at the sides of the body, by groups 

 of dorso-ventral muscular fibres. The power which the proglottis 

 possesses of altering its form is clue to the interaction of all these 

 muscles. By means of them it is able to shorten itself considerably, 

 at the same time becoming much broader and thicker, or to elongate to 

 double its normal length, becoming much thinner. In the connective 

 tissue parenchyma of the body, not only the muscles, but all the other 

 organs are embedded. In its peripheral portion, especially in the neigh- 

 bourhood of the head, we find small densely packed calcareous concre- 

 raents. which are generally regarded as calcified connective tissue cells. 



