ON PARASITES. 25 



The genital apertures are approximated. 



The perfect animal is completely hermaphrodite. 



The ovules contain embryos rarely like their parents. In the 

 miniature state their embryos wander free in the outer world or 

 remain enclosed in the parenchyma of organs, especially in the inferior 

 animals. In this miniature or larva condition these animals are 

 called cercarise. They are sexless invariably. Observation upon the 

 monostoma mutabile demonstrates, that from the eggs of the 

 Trematoda issues a simple saccular structure or organism, to which 

 the name sporocyst has been applied. In the interior of this sporo- 

 cyst, a number of second cysts are formed and in the interior of these 

 cercarise The cercaria escapes from its " nurse " or sporocyst, and 

 finding its requisite nidus for development produces a trematode 

 worm. 



The Monostoma lentis, Distoma hepaticum, Distoma haematobium 

 and Distoma lanceolatum are the Trematoda thus far found in the 

 human subject. 



The fourth order, Acanthotheca, does not occur in the human sub- 

 ject. It includes the gordiacea or hair like worms which infest the 

 frog, among other animals. I omit further mention of it and proceed 

 to the 5th order, the jN^ematoidea. This order is the highest of the 

 Entozoa in the scale of organization, as is seen from the table. The 

 muscular system being now clearly developed, a corresponding supply 

 of innervation mu'-t be provided. Accordingly nervous filaments 

 connect the various fasciculi together so as to secure greater power of 

 locomotion and prehension. The intestinal tube is limited by a 

 special membrane and is lodged along with other organs in an abdo- 

 minal cavity. The animals are, however, destitute of special senses 

 and nervous centres, so that common sensation is still their highest 

 attribute. 



Many points connected with the history of this order are still in 

 obscurity. The Trichina spiralis which occurs in the muscular system 

 of man so abundantly, is classified in this order, although without 

 sexual system. It is probably a stage of development of some one 

 of the sexual worms, and the Trichocephalus dispar is as likely as 

 any other. 



Oedee V. Nematoidea. 

 The body is rounded, attenuated more or less, thread-like, elastic, 

 with anal orifice, central or subcentral. The intestines perfect, anus 



