68 



ANIMAL STUDIES 



or crawling. They also give support to a brain, which is 

 in the form of a collar encircling the pharynx near the 

 head, and to the great nerves which extend from it. Still 

 further within the transparent body the alimentary canal 

 may be distinguished as a straight 

 tube passing directly through the 

 animal. The alimentary canal lies 

 freely in a great space, the body 

 cavity, traces of which may exist 

 in the flatworms in the form of 

 hollow spaces into which the kid- 

 neys open. It is possible that in 

 this form also the kidneys open 

 into this space, and it is roomy 

 enough besides to afford lodgment 

 for the reproductive organs in 

 addition to a large amount of fluid 

 which is probably somewhat of 

 lae nature of blood. A space in 

 rfome respects similar to this occurs 

 in all the animals above this group, 

 and as we shall see, it is often cu- 

 riously modified and serves for a 

 number of different and highly im- 

 portant purposes. In the round- 

 worms the fluid it contains proba- 

 bly acts in the nature of a blood 

 system, distributing the food and 

 oxygen to various parts of the body and carrying the wastes 

 to the kidneys for removal. 



63. Multiplication. In the matter of the production of 

 new individuals the greatest differences exist. In some 

 threadworms, for example the " vinegar eel," eggs develop 

 within the body and the young are born with the form of the 

 parent. In other cases the eggs are laid in the water, where 

 they, too, may directly grow to the adult condition ; but in 



FIG. 37. Thread- or round- 

 worms. A, vinegar eel (An- 

 guillula) ; m, mouth ; ph., 

 pharynx ; i, intestine ; ov. t 

 developing young. B, Tri- 

 china. From Nature, greatly 

 enlarged. 



