SUMMARY OF IMPORTANT FACTS. 265 



Summary of Important Facts. 



1. The CCELENTERATA (together with the Echinoderma) 

 were formerly called Radiata because in most a radial form of 

 structure is present; in the higher groups this can be transformed 

 into biradial or even bilateral symmetry. 



2. The Coelenterata are sometimes called Zoophyta (animal 

 plants), from their appearance and their attachment. In many 

 the resemblance is heightened by their formation of plant-like 

 colonies by fission and budding. 



3. The name Coelenterata was chosen because they have but 

 one system of cavities, a simple or ramified digestive sac, repre- 

 senting at once the alimentary tract and the as yet undifferen- 

 tiated body cavity. 



4. This ccelenteric apparatus is called the gastrovascular sys- 

 tem because its branches distribute nourishment to all parts and so 

 perform the function of blood vessels. 



5. The reproduction is either sexual or asexual, very frequently 

 cyclical (alternation of generations). 



6. The animals are provided with nerves, muscles, and sense 

 organs and possess marked sensibility and mobility. 



7. Especially characteristic are the tentacles and small nettling 

 organs, the cnidse, in special cells. 



8. Nearly all histological differentiation proceeds from ectoderm 

 or entoderm, since the mesoderm (mesoglcea) plays but a subordi- 

 nate role and usually functions only as a support. 



9. Four classes Hydrozoa, Scyphozoa, Anthozoa, and Cteno- 

 phora are recognized. 



10. In HYDKOZOA and SCYPHOZOA there are usually two 

 alternating generations, the sessile asexual polyp and the free- 

 swimming sexual medusa. 



11. The hydroid polyp and the craspedote medusa are charac- 

 teristic of the HYDROZOA. 



12. The hydroid polyp is a two-layered sac of ectoderm and 

 entoderm, a supporting layer and a circle of tentacles. In the 

 colonial forms there is usually a cuticular envelope, the perisarc, 

 secreted by the ectoderm. 



13. The craspedote medusa is bell-shaped, with smooth bell 

 margin, its aperture partially closed by a diaphragm-like velum; 

 the gonads are ectodermal. 



14. The medusae arise by lateral budding from the hydroid. 



15. If the medusa remain attached to the parent as a sporosac, 



