ENTOZOA. 161 



ENTOZOA (Internal Parasites), 



Parasites may be either vegetable or animal. 



Among the former may be mentioned, Achorion schonleini, Oidium albi- 

 cans, Sarcma ventriculi, Penicillium glaucum. Animal parasites chiefly 

 belong to the articulate type. Divided into Entozoa and Ectozoa. 

 Ectozoa, see pp. 151, 156, 157, 158. 

 Entozoa divided into 



( Nematoidea. 

 Helminths. -\ Trematoda. 

 (^ Cestoda. 



( Parasitic Infusoria. 

 Protozoons. -\ Psorospermiae. 

 (^ Gregarinidae. 



NEMATOIDEA. Body elastic, hollow, sub-cylindrical. Alimentary canal 

 simple. Head but slightly protractile. (Diesing.) A fine chitinous cuticle 

 invests the body to be shed twice a year. A water-vascular system exists in 

 the form of two long tubes, ending caecally at each end and traversing each 

 lateral thickening of the dermis. Toward the anterior end of the body 

 each tube gives off a small branch, and these two branches uniting together 

 open externally by a small pore. The nervous system may be said to con- 

 sist of an oesophageal ring, with three ganglia two lateral and one ventral, 

 the latter being far the largest. Nervous fibres are continued backwards 

 along the ventral side of the body to the anus ; and several nervous fibres 

 are said to run forward from the cesophageal ring. No sensory organs have 

 been discovered besides the papillae of the mouth, except in free forms, 

 which are provided with eyespots. The alimentary canal is of variable form. 

 The Nematoidea were supposed to be all dioecious by Schneider ; but the 

 Anguillidae inhabiting putrefying snails have an ova-testis, and the same 

 structure has been said to exist in an Ascaris. Males smaller than females ; 

 each has a long vas deferens opening below, in the front wall of a sort of 

 cloaca, while its caecal upper end furnishes sperm cells, the complete devel- 

 opment of the spermatozoa only taking place inside the female. The oviduct 

 of the latter is similar in form to the male tube just mentioned, and ova are 

 secreted at its caecal upper portion. (Huxley.) 



Trichina. 



11 



