PHYLUM PLATYHELMINTHES 



241 



Body-wall.- -The body- wall (Fig. 187) is found on section to 

 comprise three layers : (1) a homogenous cuticle (cut.) of which 

 the spinules (sp.) are 

 special developments ; (2) 

 a layer of circularly dis- 

 posed muscular fibres (circ. 

 mus.) ; (3) a layer of longi- 

 tudinal muscular fibres 

 (lung. mus.). A cellular 

 epidermis is wanting. 

 Beneath the muscles are 

 unicellular 



arc 



numerous 



FIG. 1ST. Fasciola hepatica. Section of the integu- 

 ment, circ. mus. layer of circular muscular fibres ; 

 cut. cuticle ; gl. unicellular glands ; long. mus. layer 

 of longitudinal muscular fibres ; sp. spinules. (After 

 Braun.) 



glands (gl.), the ducts of 

 which, in the form of pro- 

 cesses of the cells, open on 

 the outer surface. Inter- 

 nally, the interspaces 

 between the organs are 

 filled by a peculiar form of connective - tissue, the paren- 

 chyma. 



Digestive System. The mouth (Fig. 188) leads to a small 

 rounded bulb-like body, the pliarnyx (ph.), with thick muscular 

 walls and a small cavity. From this a short passage, the (esophagus, 

 opens into the intestine. The latter (int.) is frequently a very 

 conspicuous structure, owing to its being filled with the dark 

 biliary matter mixed with blood on which the Fluke feeds. It 

 divides almost immediately into two main limbs, right and left, 

 and from each of these are given off, both internally and externally, a 

 number of blind branches or caeca, those on the inner side being short 

 and simple, while those on the outer side are longer and branched. 

 The two limbs of the intestine with their branches thus form, as 

 in the Planarian, a complicated system, the ramifications of which 

 extend throughout the whole of the body. There is no aperture 

 of communication between the intestine and the exterior, the only 

 external opening of the alimentary system being through the 

 mouth. 



A branching system of vessels the water- vessels or vessels of 

 the excretory system ramify throughout the body. A longi- 

 tudinal main trunk opens behind by means of the excretory pore 

 already mentioned as occurring at the posterior end. In front it 

 gives off four large trunks, each of which branches repeatedly, the 

 branches giving off smaller vessels, and these again still smaller 

 twigs, until we reach a system of extremely fine microscopic vessels 

 or capillaries. Each of these ends internally in a slight enlarge- 

 ment situated in the interior of a large cell, an excretory cell or flame- 

 cell, similar to a flame-cell of the Planarian. 



The Liver-Fluke has a well differentiated nervous system, 



VOL. I 



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