408 The Animal Kingdom 



foods. Special cells lining the intestine engulf the food particles by 

 amoeboid movements and digest them in food vacuoles. 



Reproduction in planaria is both sexual and asexual. During 

 mating, two planarians approach and there is a brief courtship consisting 

 of body contact. During copulation, the two planarians face away from 

 one another, and the posterior ends are raised and placed together. 

 The penis of each one is inserted into the genital pore of the other. The 

 sperm are deposited from the male seminal vesicle of one copulant into 

 the female seminal receptacle of the other. After the animals separate, 

 the sperm work their way up the oviducts and fertilize the ripe eggs as 

 they come from the ovary. On the way down the oviduct the zygotes 

 and yolk cells from the yolk glands are combined. A capsule is secreted 

 around several of these, and this capsule is later fixed to some object 

 by a small stalk. After a period of time, the eggs hatch and a small 

 planaria emerges from each one. 



Asexual reproduction occurs by means of simple fission, that is, 

 the animal is able to divide into two parts. The split usually occurs 

 just behind the pharynx and the missing parts are regenerated by each 

 segment. 



Regeneration. — Regeneration is so well developed among these 

 planarians that it has been the subject of many experiments and labora- 

 tory demonstrations. A worm may be cut into many small segments and 

 if properly handled, each segment will regenerate a complete worm. 

 Regeneration will occur if the worm is cut either transversely or longi- 

 tudinally. Double-headed planarians may be produced by splitting the 

 anterior end and leaving the rest of the worm intact. If a transverse 

 cut is made directly behind the sensory lobes, a strange animal with a 

 head at both ends results. 



It is noted in these regeneration experiments that the head end tends 

 to develop first. The reason for this has been shown to be the higher 

 metabolic rate of the anterior end. There is an actual gradient of meta- 

 bolic activity along the axis of the animal. This is highest at the head 

 end. The anterior end of any cut piece will regenerate a head, while 

 the posterior portion regenerates the tail. These two portions may be 

 continguous in a cut, but the posterior part of the anterior segment 

 will develop into the tail, the anterior part of the next segment into the 

 head. Thus it is the metabolic relationships of each cut piece that de- 

 termine what develops. This axial gradient theory has been used to 

 explain many processes of regeneration and embryonic differentiation 

 in higher forms. 



