. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. TKEMATODA. 317 The excretory apparatus consists of two large lateral trunks and a network of fine vessels permeating the tissues and beginning with small ciliated lobules. The two large trunks open into a common contractile vesicle, which opens to the exterior at the posterior end of the body (fig. 253, Ep). The excretory system contains a watery fluid with granular concretions. This fluid is probably an excretory product, corresponding to the urine of higher animals. The nervous system con
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. TKEMATODA. 317 The excretory apparatus consists of two large lateral trunks and a network of fine vessels permeating the tissues and beginning with small ciliated lobules. The two large trunks open into a common contractile vesicle, which opens to the exterior at the posterior end of the body (fig. 253, Ep). The excretory system contains a watery fluid with granular concretions. This fluid is probably an excretory product, corresponding to the urine of higher animals. The nervous system consists of a double ganglion lying above the oesophagus, and from it several small nerves and two posteriorly directed lateral trunks are said to be given off. Eye sj)ots with refractive bodies are sometimes present in the larva? during their migrations. Locomotion is effected by the dermal muscular system and the organs of attachment, viz., the suckers and hooks, which present numerous modifications in number, form, and arrangement. In general, the size and development of these organs are related to the endo- parasitic or ecto-parasitic mode of life. In the endo-parasitic Trema- todes they are less developed, and usually consist of the oral sucker and a second larger sucker on the ven- tral surface, either near the mouth, as in Distomum, or at the opposite pole of the body (AmpJiistomu'ni). This large sucker may, however, be absent (Monostomuni). The ecto- parasitic Polystomea, on the other hand, are distinguished by a much more powerful armature, for besides two smaller suckers at the sides of the mouth, they possess one or more large suckers at the posterior end of the body (fig. 258), which, moreover, may be supported by rods of chitin. There are often in addition chitinous hooks, and very frequently two larger hooks among the posterior suckers in the middle line (//). The Trematoda are mostly hermaphrodite. As a rule, the male and female generative openings lie side by side,
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