288 THE ANIMAL KINGDOM 



gastrulation, and that none of the other cells are able to produce meso- 

 derm or endoderm. 



The work oi VV^ilson, Costello and many others leads to the same 

 general conclusions: In the early development of annelids and molluscs 

 the abilities of the parts of the embryos are limited to the functions 

 they serve in normal development (with a few exceptions in which some 

 portions are able to form a lew additional structures). A second and 

 equally significant conclusion is that in some cases these abilities can 

 be realized in isolation, without interaction among the parts. Examples 

 are the cilia of the apical organ and of the prototroch that developed 

 in Costello's isolates. It should be added that the development of other 

 structures appears to require the integrity of the embryo, since the 

 macromeres in Costello's experiments showed no tendencies to form 

 mesoderm bands or digestive tract, and none of the ectomesodermal 

 structures appeared in any of his isolates. 



Questions 



1. Discuss segmentation in the animal kingdom. 



2. Draw cross sections of a polychaete and an oligochaete. 



3. Compare the sense organs of Nereis and Lumbricus. 



4. Describe a parapodium. 



5. Discuss the role of giant fibers in annelids. 



6. How can a population achieve reproductive coordination so that all individuals breed 

 at one time in the year? 



7. How do leeches feed? 



8. Compare reproduction in Nereis, Lumbricus and a leech. 



9. Draw and label a trochophore. 



Supplementary Reading 



The photographs and stereodiagrams of annelids in Buchsbaum, Animals Without 

 Backbones, are especially good. The colored photographs and life studies of annelids in 

 Vonge, The Sea Shore, are excellent. The development of the trochophore and of other 

 larvae are discussed in Willier, Weiss and Hamburger, The Analysis of Development. 



