NO. 3 INSECT HEAD— SNODGKASS 5 



This is a satisfactory explanation of the origin of the gastrula if 

 not questioned too closely; but Bidder (1927) rather disturbs the 

 idea with his statement that " the laws of viscous matter make it clear 

 that the free-swimming gastrulae we observe as larvae could never 

 earn their own living, since the stream-lines would carry every particle 

 of food outside the cone of dead water which is dragged behind the 

 gastrula mouth." On the other hand, Bidder admits, '" creeping 

 planulae or gastrulae might pick things up." A creeping animal, 

 however, would never in the first place develop a mouth at the rear 

 end of the body. What we want is an explanation of the original 

 posterior position of the blastopore, and if none offered will sufifice. 

 we must be content with the fact. 



The further history of the coelenterate larva has no bearing on the 

 evolution of insects, for the creature soon becomes attached by its 

 head end, and. probably as a result of the sedentary, plant-like habits 



Fig. 3. — Free-swimming plaiuila larva of a coelenterate, Sympodiuin corral- 

 1 aides, with ciliated ectoderm, and completeb'-formed endoderm. (From Ko- 

 walevsky and Marion, 1883.) 



of its immediate ancestors, develops into a polype or jellyfish having 

 a radiate, flower-like type of structure. Some writers have suggested 

 that the worms and the arthropods may have been evolved from an 

 elongate medusa, but it seems more probable that the Coelenterata, 

 the Annelida, and the Arthropoda are all to be traced back to a free- 

 swimming gastrula ancestor. The mature planula is a specialized 

 gastrula, but it is of general interest in that it gives us a ])assing 

 glimpse of a free-living animal in the blastula and gastrula stages at 

 a time when cephalization was first established in the Metazoa. 



The structure and development of the arthropods suggest that 

 these creatures were developed from forms adapted to a creeping 

 rather than a swimming mode of progression. Some planula larvae 

 lack cilia and have creeping habits, and such forms, though they have 

 nothing to do with the arthropod ancestors, show that a free-living 

 creature in the blastula or gastrula stage may change its mode of 

 ])rogression. The creeping habit as an habitual mode of progression 

 entails some fundamental structural adaptations. An animal that crawls 



