150 HELMINTHOZOA. 



be regarded as so many loose ovaries. When the eggs have reached a 

 certain size, they fall from the ovaries into the cavity of the body, where 

 they continue to increase in size, and become enclosed in additional 

 envelopes. "When mature, the ova escape through a muscular canal, 

 which terminates immediately at the vulva, the latter being a simple 

 aperture, situated at the posterior extremity of the worm. The muscular 

 canal, through which the eggs escape, is of a campanulate or infundi- 

 buliform shape, opening internally by an aperture whose borders float 

 freely in the cavity ; of the body and thus the whole apparatus might be 

 compared to a Fallopian tube. 



(396.) The generative system of the male Echinorhynchus is repre- 

 sented in fig. 74, 2. The organs which secrete the fecundating fluid 

 are two cylindrical vesicles (/, g), attached at one extremity by minute 

 filaments to the walls of the body ; from each of these arises a duct (ft), 

 and the two, uniting at i, form a common excretory canal. This canal 

 speedily dilates into a number of sacculated receptacles, in which the 

 secretion of the testes accumulates ; and from them a duct leads to the 

 root of the penis (m). The penis or organ of copulation, when extended, 

 protrudes through the aperture (p) placed at the anal extremity of the 

 body; but when retracted, it is folded up and lodged in a conical 

 sheath (o). The protrusion and retraction of this part of the male appa- 

 ratus is effected by a very simple mechanism : two muscles (I I), arising 

 from the inner walls of the body, are inserted into the base of the 

 sheath (m), and serve to draw it inwards ; and two others (n n), in- 

 serted at the same point, but arising from the posterior extremity of the 

 animal, by their contraction force outwards the copulatory organ an 

 arrangement precisely corresponding with that by which the movements 

 of the proboscis are provided for. 



(397.) The TTTRBELLARI^E, constituting another important group of 

 the Helminthozoa, are mainly characterized by having the exterior of 

 their bodies densely covered with vibratile cilia, by the agency of which 

 they swim freely about. They are not parasitical in their habits, and 

 are met with under various forms, both in the sea and in fresh water. 

 They may be divided into two families, the PLANARI^E and the NEMER- 

 TIAN WORMS. 



(398.) The PLANARI^E, although they do not inhabit the interior of 

 other animals, are nearly allied in every part of their organization with 

 the Flukes (Distoma) ; so that their history cannot be more appropriately 

 given than in this place. The Planarise are common in ponds and other 

 stagnant waters ; they are generally found creeping upon the stems of 

 plants, or amongst the healthy confervae which abound in such situa- 

 tions, and wage perpetual war with a variety of animals inhabiting the 

 same localities. The body of one of these minute creatures appears to 

 be entirely gelatinous, without any trace of muscular fibre * ; yet its 

 * Duges, Aim. des Sc. Nat. 1828. 



