246 



PLATYHELMINTHES. 



The simplicity of the internal organization corre- 

 sponds with the simple appearance of the external 

 structure. Like the Trent ato< la, the Cestoda are 

 said to possess no epithelial ectoderm. Beneath 

 the cuticle-like outer membrane is a layer of 

 spindle-shaped cells lying at right angles to the 

 surface ; their external ends abut upon the cuticle, 

 and their inner ends are prolonged as fibres into 

 the parenchyma. Beneath this layer there is a 

 delicate superficial layer of longitudinal muscular 

 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 due 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 concretions, which are prob- 

 ably contained in connective tissue cells. 



The nervous system consists of two 

 lateral longitudinal cords passing exter- 

 nally to the main trunks of the excretory 

 system. They are somewhat swollen in 

 the head, where they are connected by a 

 transverse commissure ; these anterior 

 swellings and the commissure may repre- 

 sent a cephalic ganglion. In Moniezia 

 the lateral nerves are connected by two 

 transverse commissures at the hind end 

 of each proglottis. Distinct sense-organs FlG- i 99 ._Head of a Taenia with 

 are wanting, but the tactile sense may be the four suckers, and the con- 



necting loops of the excretory 



ascribed to the skin, especially to that ot cana i (after Pintner). 



FIG. 198. Young 

 Tetrurhynchus with 

 beginning segment- 

 ation. The four ex- 

 cretory canals with 

 the connecting 

 loops in the head, 

 and the terminal 

 vesicle B are visible 

 (from Glaus). 



