

TREMATODA. 643 



and sometimes soniferous. The spines are occasionally very conspicuous, 

 e. g. on the body of Fasciola hepatica^ on the oral extremity of some species 

 of Distomum^ and especially on the body and round the oral sucker of 

 Echinostomum. The cuticle is prolonged inwards at the apertures of the 

 different organs opening externally, and it is said in various instances 

 to be underlain either by a layer of cells, by a granular layer, a layer of 

 elastic fibres, or by the circular layer of body muscles 1 . Unicellular 

 cutaneous glands are said to be present in some instances, e.g. on the 

 posterior sucker of Tristomum, on the surface of the body in Polystomum 

 integerrimum and Aspidogaster, &c. Glands open on the adoral processes 

 of GyrodactyluS) and on the lateral lobes of Holostomum, and probably 

 secrete an adhesive material. The musculature of the body consists 

 typically of an external layer of circular fibres, followed by a layer of 

 longitudinal fibres, and this in turn by diagonal fibres, crossing from right 

 to left and vice versd. But there are variations from this typical arrange- 

 ment. Bundles of dorso-ventral fibres pass vertically from the dorsal to 

 the ventral surface, and their ends are attached to internal prominences of 

 the cuticle. Special bundles of muscles are sometimes present in con- 

 nection with the oral sucker, and the terminations of the genital organs. 

 Cells disposed in groups or in a layer, are often present internally to the 

 muscular layers. Their significance is not known. The suckers are limited 

 internally by a distinct membrane, and are composed of muscular fibres, 

 arranged equatorially, meridionally, and radially, the last-named constitut- 

 ing the bulk of the organ. The substance of the body is made up of 

 a tissue variable in character ; either simply cellular ; or composed of 

 a cell-network in which the cells are more or less distinct with other cells 

 of a rounded aspect contained in their meshes ; or consisting of a matrix 

 imbedding distinct or feebly indicated cells and fibres. Spaces between 

 the cells or in the matrix have been held to represent the coelome. 



The nervous system consists of a pair of ganglia placed anteriorly, 

 and connected by a transverse commissure dorsal to the pharynx. A 

 subpharyngeal ganglion, connected by two commissures with the supra- 

 pharyngeal ganglia, has been described in Fasciola hepatica (Sommer), 

 and a pair of sub-oesophageal nervous commissures in Distomum isostomum 

 (Gaffron). Two pairs of nerves usually pass forwards from the ganglia, 



1 Kerbert found in Distomum Westermnani (i) a delicate cuticle, (2) a layer of cells, (3) a 

 basement membrane to which the spines were attached, (i) and (2) were sometimes missing. 

 Hence he concludes that the Trematode cuticle = a basement membrane, A. N. A. xix. 1881, p. 531. 

 But Ziegler is convinced (Z. W. Z. xxxix. 1883, pp. 542-7) that the cuticle is a metamorphosed layer 

 of cells. Nuclei are to be found in it in Bucephalus, according to him. So too in Sporocysts and 

 Cercariae, according to Biehringer (Arb. Zool. Zoot. Inst. Wurzburg, vii. 1885, p. 4). Schwarze 

 agrees in the same view (Z. W. Z. xliii. 1886), and, according to him, the cuticle of the oesophagus, 

 excretory vesicles, and main excretory canals are similarly derived. The walls of the oviduct in some 

 . Distomidae are cellular. 



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