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 Discomedusm consists of 
this mesogdsea. In the mesogdsea 
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 
stages in development of Monoxenia darwim 
(g), the eight hollow tentacles (highly magnified). 
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 
