THE SEA ANEMONES AND CORALS 



347 * 



Kowalevski, found that many different classes of animals, in developing from 

 the egg, passed through such a stage. Haeckel, generalizing from these facts, 

 invented his Gastrea theory, according to which all animals in which the gastrula 

 stage occurs must have been descended from a common primitive form, Gastrea, 

 which has, however, in its simplest form long been extinct, but of which the 

 Coelenterates are the closest modern representatives. 



The gastrula of Monoxenia is of the simplest kind, the infolding being com- 

 plete, and the larva forming a sac, whose walls consist of two layers of cells, or 

 germinal layers, an outer ectoderm and an inner endoderm (see section given in the 

 illustration). The transition from the flat dish shape (H) to the sac with a narrow 

 mouth is at once clear, and the knowledge that all the Coelenterates proceed from 

 a similar larva, and that all the complications of their various systems are 

 developed from such a simple gastrula, throws much light on their anatomy. During 

 these transformations, the en- 

 doderm, whose cells multiply, 

 continues as an uninterrupted 

 lining to the stomach and its 

 appendages, while the ectoderm 

 yields the constituents of the 

 skin. A third intermediate gela- 

 tinous layer, ihemesog/cea, arises 

 between the outer and inner 

 layers; in this, muscles and 

 connective interstitial tissues 

 appear. The chief part of the 

 jelly forming the great umbrella 

 of the Discornedusse consists of 

 this mesoglaea. In themesoglaea 

 of one division of the corals the 

 calcifications take place. These 

 internal calcifications play, how- 

 ever, but a very small part in 

 the great rock-making activities 

 of corals as a whole, the most 

 important calcifications being ex- 

 ternal. Returning to Haeckel 's 

 account of Monoxenia, although 

 the transition from the gastrula 

 larva to the adult animal has 

 not been observed, there can be 

 no doubt as to how it takes 

 place; all the transformations 

 having been watched in other 



species. The larva attaches it- STAGES IN DEVELOPMENT OP Monoxenia darwini. 

 self with the end opposite to the (Highly magnified.) 



