366 THE ANIMAL PARASITES OF MAN 



In the male each ventral part of the lateral nerves becomes 

 thickened by taking up fibres from the ventral nerves, which become 

 thickened posteriorly to the nervus bursalis, which towards the 

 middle gives off a mass of fibres to the " genital papillae " situated 

 in front of and behind the anus ; the number of these fibres averages 

 eighty to 100 ; in its further course the bursal nerve resembles the 

 corresponding ventral part of the lateral nerves of the female. 



The ventral and dorsal nerves are connected by a number of 

 semicircular commissures, which originate from the ventral nerves 

 and serve to supply the dorsal nerve, which is always being 

 decreased by fibres departing from it. It is remarkable that these 

 commissures are not placed symmetrically, and their position also 

 is different in the two sexes ; in the female there are thirty-one on 

 the right side and only thirteen on the left side. In the male there 

 are thirty-three commissures on the right side and fourteen on the 

 left, which run into the subcuticular layer, generally in pairs, and 

 usually cross at the level of the lateral lines. 



The fibres of the two median nerves are chiefly motor ; fascicular 

 processes run from each protoplasmic part of the muscular cells to 

 the median nerves ; from these they take up bundles of primitive 

 fibrils, which separate, pass through the protoplasmic part and enter 

 the contractile part (fig. 260). One part of the fibrils, however, 

 penetrates beyond the muscles into the subcuticular layer, where 

 they form a network, probably of a sensory nature, with contiguous 

 fibrils. Nerves directed anteriorly finally originate from the 

 cesophageal ring; they consist each of three fibres, carry three 

 ganglion cells at their point of origin, and enter the sensory organs 

 of the three papillae surrounding the oral aperture. Two of these 

 little trunks lie in the lateral lines, the remaining four are situated 

 in the middle of the four quadrants (Nn. sub-mediani anteriores). 



Parasitic species lack higher ORGANS OF SENSE ; free-living worms 

 occasionally have two rust-red eyes, sometimes with lenses, at the 

 anterior part of the body. In addition to the above-mentioned 

 sensory papillae surrounding the oral aperture and the genital papillae 

 of the male at the end of the body, another pair exist in the vicinity 

 of the lateral ganglia, the " cervical papillae," and two dorsal papillae 

 in the central region of the body and two lateral ones near the tip of 

 the tail (Ascaridce). The differences in the distribution and number 

 of the sensory papillae serve for characterizing the larger and smaller 

 groups of Nematodes. 



THE EXCRETORY ORGANS of the Nematodes are variable. In a 

 great many cases the apparatus is symmetrical, and consists of a vessel 

 commencing in the posterior extremity in each lateral line (fig. 260), 

 and passing anteriorly. In the vicinity of the anterior extremity both 



