SEA-ANEMONES AND CORALS. 



497 



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

 these transformations, the endoderm, whose cells multiply, continues as an uninter- 

 rupted lining to the stomach and its appendages, while the ectoderm yields the 

 constituents of the skin. A third intermediate gelatinous layer, the mesoglcea, 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 Discomedus89 consists of 

 this mesoglsea. In the mesoglaea 

 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- 

 self with the end opposite to the 

 mouth, the cilia disappear, and 

 after the mouth -tube (p) has 

 been formed by the folding in 

 of the anterior end along the 

 longitudinal axis of the body 

 (L, o, a), and has thus become 

 marked off from the stomach 

 (#), the eight hollow tentacles 

 rise round the mouth as out- 

 growths of the body-cavity, or as direct continuations of the stomach. Like all other 

 corals, Monoxenia periodically multiplies by means of eggs which arise either in 

 the walls of the radiating stomach partitions (or septa), or on their free edges, and 

 have to be ejected through the mouth, as development does not in this case take 

 place within the digestive cavity of the parent polyp. As a rule, the polyps are 

 either male or female, but in stock-forming species individuals of the two sexes may 

 be mixed. Hermaphrodite individuals are less frequent. 



Monoxenia may be taken as the simplest type of the regularly radiate polyps ; 

 in all radiate animals the different organs being repeated in regular rings round 



VOL. VI 32 



STAGES IN DEVELOPMENT OF Monoxenia darwini 

 (highly magnified). 



