PHYLUM PLATYHELMIA 177 



then breaks away from the anterior portion. New in- 

 dividuals are thus produced by transverse fission. 



Planaria also carries on reproduction by sexual processes 

 involving the fusion of egg and sperm cells. Each individual 

 is a hermaphroditic and, therefore, has complete sets of both 

 male and female organs (i.e., testes, vasa deferentia, or sperm 

 ducts, penis; and ovaries, oviducts, sperm receptacle, yolk 

 glands, shell glands) . Though a planarian might ' i fertilize ' ' 

 itself by injecting its own spermatozoa into its female repro- 

 ductive system, it does not commonly do so, but obtains its 

 supply of sperm from some other individual, and in turn 

 gives its sperm to fertilize the other party concerned in the 

 mating. The eggs laid by Planaria are of two types, but 

 both have several yolk cells enclosed in the shell with the 

 ovum. During the summer thin-shelled eggs are laid which 

 develop quickly, but in the autumn thick-shelled "winter 

 eggs" are produced which lie dormant until spring. 



The cleavage and gastrula stages in Planaria' s develop- 

 ment are somewhat peculiar but there is general agreement 

 with the stages usual in metazoans (Fig. 64). Though 

 Planaria is like a gastrula in its retention of the single 

 primitive opening into the enteron, it is specialized in many 

 other respects. There is a large mass of mesoderm which 

 consists mostly of muscles and loose parenchyma cells 

 (Fig. 76) ; the systems of organs are much better organized 

 and more elaborate than in ccelenterates, particularly the 

 reproductive, excretory and nervous systems. Specializa- 

 tion toward better things than have been suggested in the 

 structures of the sponges and coelenterates are shown by 

 Planaria (1) in the assumption of the bilateral type of 

 symmetry, and (2) in cephalization. Most of the dominant 

 and highly specialized animals on the earth are bilaterally 

 symmetrical ; in fact, radial symmetry may usually be taken 

 as a sign of racial conservatism, or even of retrogression. 

 Cephalization, or the placing of important organs at the 

 anterior end, naturally follows the acquisition of bilateral 

 symmetry. A jellyfish may move in any direction and 



