2l6 THE ANIMAL PARASITES OF MAN 



Of the muscles of the organs which have developed from the 

 parenchyma muscles we may briefly mention those bundles that are 

 attached to certain parts of the genital apparatus, to the suckers, 

 to the hooks and claws, and also, at all events in Fasciola hepatica, 

 to the spines. The sheaths used for the projection of the tentacles 

 of the Rhopaliadce are also muscular. 



The contractile elements consist of fibres of various lengths 

 that are mostly parallel to one another, and frequently anastomose ; 

 a cortical substance finely fibrillated can usually be distinguished 

 from an internal homogeneous mass ; large nucleated cells of uniform 

 size are always connected with them ; these have been variously 

 interpreted, but have been proved to be myoblasts, one or more 

 of their processes constituting the muscular fibres. 



The MOVEMENTS of the Trematodes consist in alterations of form 

 and position of the body, as well as in creeping movements. 



In the NERVOUS SYSTEM (fig. 121) can be distinguished a cerebral 

 portion as well as strands (medullary strands) running from it, 

 and peripheral nerves. The cerebral portion always consists of 

 two large ganglia situated in the anterior end of the body which 

 pass dorsally over the oesophagus and are connected by means 

 of a broad and thick commissure composed of fibres only. From 

 each ganglion three nerves run anteriorly the inner and dorsal 

 nerve for supplying the anterior dorsal part of the body ; the 

 median and ventral for the oral sucker; and the exterior and lateral 

 likewise for the supply of the sucker. 



In a similar manner three strands run backwards from each 

 ganglion one dorsal, one lateral and one ventral. The dorsal and 

 ventral strands become united and curve backwards ; the symmetrical 

 lateral strands are connected by means of transverse commissures, 

 the number of which vary according to the species. Such commis- 

 sures also exist between the lateral and the two other strands on each 

 side. There are ganglion cells along the entire course of the posterior 

 cords, more particularly at the points of origin of the commissures. 

 There also appears to be in addition a fourth anterior and posterior 

 pair of nerves, the front pair for the oral sucker and the hind pair 

 for the pharynx. 



The peripheral nerves, which spring from the posterior strands 

 as well as from the commissures, either pass directly to the muscular 

 fibres or to the sensory cells that are situated at the level of the 

 subcuticular cells, or they reach these after the formation of a plexus 

 situated immediately beneath the dermo-muscular layer ; the pro- 

 cesses directed outwards terminate in small vesicles in the cuticle. 



As to other ORGANS OF SENSE, simple eyes, two or four in 

 number, are known in several ectoparasitic species as well as in a 



