^^^ ZOOGENESIS f^^^ 



tance called the coelom. The coelom has three divisions. These 

 three divisions are: Firsf, the perivisceral, which forms the body 

 cavity in which the heart and other viscera lie; second^ the gona- 

 dial, or reproductive portion, from the walls of which the repro- 

 ductive cells arise; and third, the nephridial, the walls of which 

 secrete the nitrogenous waste. 



It is impossible not to see in the three divisions of the coelom 

 a correspondence with the three distinct types of polyps produced 

 by many of the colonial coelenterates. It is likewise impossible 

 not to see in the extensive asexual reproduction by budding in 

 the flukes the same phenomenon as the extensive asexual repro- 

 duction in the coelenterates. In the coelenterates the buds which 

 grow into the new individuals — polyps — are always external, 

 while in the flukes they are always produced within the original 

 unit. 



It is quite conceivable that the coelom may have arisen from 

 the budding internally as in the flukes instead of externally as in 

 the coelenterates of a sack-like, a reproductive, and an excretory 

 unit corresponding to each one of the three types of polyps 

 characteristic of many colonial coelenterates. Such an explana- 

 tion of the origin of the coelom is at least plausible on the basis 

 of the evidence, and no other explanation which does not involve 

 the creation of a new structure out of nothing is possible. 



3. The solitary flatworms and roundworms, wholly independ- 

 ent of each other, each individual developing directly, or through 

 a larval form, from an egg without any asexual reproduction. In 

 this group fall the thread-worms or nematodes (fig. 81, p. 161), 

 most turbellarians (fig. 54, p. 97), and some other types. 



4. Flatworms which are independent of each other, but form 

 colonies of similar perfect individuals through asexual reproduc- 

 tion, as in the case of Microstomum (fig. 135, p. 103). 



Thus we find in the Vermiformes four main structural types. 

 These four chief structural types are : 



I. Mainly bilateral animals taking the form of a linear and 

 more or less unified colony. 



z. Mainly bilateral animals in which colony formation is in- 

 verted, the budding off of the new elements taking place within 

 the original unit. 



3. Mainly bilateral independent animals which do not form 

 colonies and which show no asexual reproduction. 



t53] 



