112 AN INTRODUCTION TO THE STUDY OF FOSSILS 



ing of three layers : the ectoderm, the mesogloea and the endo- 

 derm, the latter lining the digestive cavity. It is of special 

 interest that this adult animal should persist in the shape of the 

 gastrula, since all higher animals pass through this gastrula 

 stage very early in their development from egg to maturity. 

 This fact is taken to mean that all higher animals have diverged 

 from a coelenterate ancestor in remote geological ages. 



1. Examine the specimens mounted in alcohol, noting the 

 size of a colony, its attachment, the individuals, each with its 

 circle of arms surrounding the mouth. 



2. Sketch, from the sHde under the compound microscope, 

 one individual animal and its connection with the central canal 

 of the colony. Label tentacles, mouth, cup, central canal. 



3. Indicate the portion of an animal capable of being pre- 

 served in the fossil state. 



4. How does the animal capture its food? 



5. What prevents a large, active animal swallowed by the 

 polyp from struggling and thus tearing the soft body walls to 

 shreds ? 



6. How is the food digested ? How assimilated ? 



7. How does the polyp breathe? 



8. What kind of a muscular and nervous system does it 

 possess ? 



9. How does it multiply ? 



10. Trace the development of an individual from the fer- 

 tilized egg to adulthood. 



11. What are medusae ? 



12. Why is a colony of Sertularia, which has a plant-like 

 attachment to foreign objects, placed with the animals? 



General Sur\^y of Class Hydrozoa 



Aquatic animals with the body consisting of a large, centrally 

 placed, digestive cavity with but one opening (the mouth) ; 

 this mouth is usually surrounded by many arms (tentacles). 



These are lowly forms of animal life with the cells making up 

 the body arranged in two layers, the ectoderm and endoderm, 

 separated by a non-cellular layer, the mesogloea. The endoderm 

 lines the large, undivided digestive cavity. 



