ANIMAL PARASITES. 3 



may be destined, in the course of their development, to pass a 

 portion of their lives in water, or perhaps (as in the case of the 

 Cercaria) as Epizoa upon other animals. Sexual organs may 

 always be detected in those specimens of these animals which have 

 attained their last grade of development. Although Oscar Schmidt 

 states that he has recently discovered a production of the six-hooked 

 brood in the Tcenia dispar of the frog, without being able to find 

 sexual organs in the segments of this animal, yet this statement 

 requires a more careful revision, especially as these organs, and 

 particularly the male ones, are so delicate that they are very 

 easily overlooked. 



The senses of sight, smell, and taste are wanting to all these 

 animals, but their general sense of touch appears to be highly 

 developed, although no undoubted proof of the existence of a 

 special nervous system in the Helmintha has yet been obtained, 

 except, in the Oxyurides, in which George Walter has recently dis- 

 covered a highly developed nervous system. The intestinal canal 

 is wanting in the Cestoidea. In the Trematoda it forms a ceecal 

 canal, in which the mouth also performs the functions of the 

 anus; whilst in the Nematoida it becomes a complete alimen- 

 tary canal, with a mouth, oesophagus, stomach, intestine, and 

 anus. In the tissues of the Cestoidea the tendeitcy to the for- 

 mation of a bony envelope makes its appearance, in the deposition 

 of calcareous salts, which have been recently regarded as siliceous 

 by Eschricht, but certainly incorrectly ; in the Trematoda we 

 generally look for these in vain, and in the Nematoidea they are 

 always deficient. If we bring together the particulars which are 

 characteristic of all these three kinds of true Helmintha, we find 

 them to comprise (besides the greatly developed general sense of 

 touch) the vascular system, consisting, in the Cestoidea, of four 

 longitudinal, lateral canals, and in the Trematoda of a fine net- 

 work of vessels, which is less distinctlv marked onlv in the 

 Nematoidea ; the muscular system, composed of transverse and 

 longitudinal muscles, without transverse stria? ; the structure of 

 the epidermis, which consists of a homogeneous, more or less 

 finely checked substance, which is, perhaps, composed of 

 chitine, but at all events of a substance which closely ap- 

 proaches chitine in its chemical reactions ; the property of 

 sooner or later giving off a strongly refractive, albuminous sub- 

 stance, in oleaginous drops, when in contact with water, and the 

 circumstance that they can scarcely ever effect their development 



