PLA TYHELMINTHES 97 



termed 'proglottides, which increase in size as they recede from 

 the head. The hody of a tapeworm may be divided into three 

 regions : (i.) the head, (ii.) the neck, and (iii.) the segmented 

 trunk. 



The head of T. saginata (Pig. 66) is spherical in shape, and 

 bears on its sides four well-developed suckers. Other species, 

 as T. solium, in addition to the suckers, are provided with a 

 double circlet of chitinoid hooks, which assist the suckers to 

 attach the worm to the inner surface of the alimentary canal 

 (Fig. 66, C) of its host. The head is mobile, and can shift its 

 point of attachment with ease. 



The neck is the region immediately succeeding the head ; 

 its most anterior half is not segmented. The first trace of 

 division into segments is the appearance of shallow grooves 

 which separate the various proglottides one from another. 

 As they grow backward the proglottides increase in size, and 

 those situated a foot or more behind the head are sexually 

 mature. 



The surface of the body is covered by a thin clear struc- 

 tureless cuticle, the layer of cells beneath this, corresponding 

 with the ectoderm of other animals, is composed of long-tailed 

 cells, the tails running down into the parenchyma. The body 

 of the tapeworm is practically solid, the coelom being repre- 

 sented by poorly-developed splits in the parenchyma. 



The muscle fibres are arranged in longitudinal trans- 

 verse and dorso-ventral bundles. The outermost layer, the 

 longitudinal, is not a very definite layer, but consists of a 

 number of unstriated fibres scattered through the parenchyma. 

 The transverse muscles he immediately within the longitudinal ; 

 they serve to divide the parenchyma into a central and a 

 cortical portion. The dorso-ventral fibres run from one sur- 

 face to the other, and are very irregularly arranged. The 

 muscle fibres are non-striated, and often branched at their 

 ends. The animal has very considerable powers of extension 

 and contraction. 



The parenchyma is composed of ill -defined connective 

 tissue cells, amongst which are scattered, especially in the 

 cortex, a number of ovoid calcareous corpuscles (Fig. 67), 

 about whose function little is known. They have been vari- 



7 



