142 SYNOPTIC COLLECTION. 



whose eggs develop into fixed or stationary hydroids, there- 

 by illustrating the process of indirect development and the 

 alternation of generations. 



The eggs of other medusae develop into medusae, skip- 

 ping altogether the hydroid stage and illustrating the proc- 

 ess of accelerated development. 



Lastly, certain hydroids whose ancestors, we have 

 reason to believe, produced free medusae have taken on 

 such reduced characters it seems probable that the me- 

 dusa stage is omitted in their ontogeny, and, if so, they 

 illustrate suppressed development. 



Acceleration in development is shown by the Disco- 

 phora, while specialization of structure and function, re- 

 sulting in complex colonial life, is. characteristic of the Si- 

 phonophora. 



The Ctenophora constitute one of those interesting syn- 

 thetic types whose relationships reach out beyond the 

 limits of the Hydrozoa, beyond even the boundaries of 

 the Coelentera, to the subkingdom of the Echinoderms. 



The ancestors of the Alcyonarian branch of the Antho- 

 zoa are illustrated by a series of forms which show most 

 admirably the gradual transition from the primitive and 

 simple to the secondary and complex. 



The living Alcyonaria have in addition to what the Hy- 

 drozoan possesses, an internal bag and a body cavity 

 divided into chambers by eight bilateral mesenteries. 

 Their skeleton is made of spicules and is chiefly a secre- 

 tion of the ectoderm of the basal membrane of the zoons. 



The Zoantharia likewise have an internal bag and in 

 youth a body cavity divided by eight bilateral mesenteries, 

 but in maturity this cavity is divided by numerous bira- 

 dial mesenteries. 



The skeleton of the Madreporaria is a solid secretion of 

 the ectoderm and consists, speaking generally, of a basal 

 disc and a theca with true septa. 



The processes of reproduction budding and fission 

 and the influence of physical forces have brought about 



