286 WILLIAM A. HASWBLL. 



The external muscular layer (c. m.) is a thin stratum of 

 circularly arranged fibres, not more than one or two fibres in 

 thickness. It is situated in a stratum of finely fibrous matter 

 containing a layer of pigment cells, which in T. fasciatus 

 on the dorsal side form with their anastomosing processes a 

 dense and regular network ; this pigment- bearing stratum is 

 much more strongly developed on the doral side than on the 

 ventral. 



The internal layer of muscle (/. m.), the fibres of which run 

 longitudinally, is much more strongly developed than the 

 external layer, and is composed of thicker fibres ; it is most 

 powerfully developed on the ventral side, but extends over the 

 whole surface and is specially well developed in the tentacles 

 on the ventral side. The fibres on the dorsal side are arranged 

 in bundles, separated from one another by very narrow inter- 

 spaces occupied by interstitial fibrous matter containing pig- 

 ment ; on the ventral side they are arranged in thin fasciculi. 



The muscular fibres of these two layers are usually angular, 

 sometimes oval in cross section, '004 mm. in diameter in the 

 internal layer, rather less in the external. They are finely 

 striated longitudinally, non-nucleated, and are separated from 

 one another by finely fibrillar interstitial matter. In cross 

 section they are seen to consist of two substances, one forming 

 a narrow darker central core, the other clearer and constituting 

 the principal bulk of the fibre. 



The nervous layer is a thin layer of the parenchyma, dis- 

 tinguished by the presence in it of a considerable amount of 

 pigment, and by being the seat of the superficial nerve-plexuses. 

 It passes insensibly into the general parenchyma of the centre 

 of the body. 



In the structure of the body wall Temnocephala resembles 

 the marine ectoparasitic Trematodes such as Tristomum 

 and Onchocotyle, and differs essentially from the Dis- 

 tomidse in the presence of the pore-canals and the absence of 

 distinct cell-boundaries in the epidermis ; the inner cell-layer 

 described by Sommer 1 in the liver-fluke is not represented, 

 1 " Die Anatourie des Leberegels," ' Zeitschr. f. wiss. Zool.,' xxxiv Band. 



