[Illustration: FIG. 121.--_Harmostomum leptostomum_, Olss., an immature specimen from _Helix hortensis_. _Nervous system_, according to Bettendorf. _A.s._, ventral sucker; _C.g._, cerebral ganglion; _Ex.p._, excretory pore; _G.p._, genital pore; _O.s._, oral sucker; _M.d._, dorsal medullary nerve; _M.l._, lateral medullary nerve; _N.ph._, pharyngeal nerve; _M.v._, ventral medullary nerve. Magnified.] 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 _Rhopaliadæ_ 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 œsophagus 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 commissures 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.