INVERTEBRATA. 



367 



the length, however, is indefinite, for the posterior proglot- 

 tides as they mature become successively detached, and 

 are expelled to the exterior. 



2. Structure. The head (fig. 189) is quite small and 

 somewhat globular : the central termination forms a 

 muscular projection called the rostellum, which is sur- 

 rounded by a ring of chitinous hooks. 

 On the sides of the thickest part of 

 the head are four muscular suckers at 

 equal distances from each other. Be- 

 hind this the head narrows to the neck. 

 The hinder part of the neck shows slight 

 transverse constrictions, which separate 

 segments from each other, the anterior 

 much less in length than in breadth, 

 but as we pass backwards the segments 

 become longer as well as broader, and 

 flatten till we reach the mature pro- 

 glottides at the posterior end of the 

 body. The number of proglottides 

 reaches 850. 



The structure of the body resembles 

 closely that of Trematodes. In a 

 transverse section is seen on the outer 

 side a cuticle; there is no regular 

 epidermis, but beneath the cuticle is a layer of spindle- 

 shaped cells with large granular gland-cells scattered in it, 

 and longitudinal muscle fibres passing through it. Beneath 

 the cellular layer is a layer of longitudinal muscle fibres, and 

 internally again a parenchymatous tissue traversed by 

 bundles of longitudinal muscles, and a layer of circular 

 muscles : there are also, especially at the borders of the pro- 

 glottides, groups of dorso-ventral fibres. All these muscle 

 fibres give the proglottids a great power of contraction in 

 all directions. Scattered in the parenchyma, especially in 

 the cortical region, are calcareous corpuscles. 



The other organs which are embedded in this muscular 

 and connective tissue are the excretory tubes or nephridia, 

 the nerve cords, and the complicated reproductive organs. 



189. HEAD OF Taenia 



Solium MAGNIFIED, 

 SHOWING THE ROSTEL- 

 LUM WITH ITS CHITI- 

 NOUS HOOKS AND THE 

 FOUR SUCKERS. 



