496 



CCELENTERA TES. 



cance. Wherever these often minute animals settle, they build up great masses of 

 rock which may form part of the solid ground of the globe. Although Aristotle 



and his contemporaries recognised the sea- 

 anemones as animals, almost two thousand 

 years elapsed before corals were considered to 

 be related to them. In describing the develop- 

 ment of a small coral discovered on the Arabian 

 coast, and named Monoxenia darwini, Haeckel 

 states that the polyp, which is one-eighth of 

 an inch long, is of strictly radiate structure, the 

 mouth, which lies at the upper end of the 

 cylindrical body, being surrounded by eight 

 feathered tentacles. It is attached to some 

 substratum by means of a flexible disc at the 

 opposite end of the body to the mouth. It is 

 clear that it has no hard skeleton, as the 

 shape of its surface is changeable ; and its 

 internal structure must be shown by transverse 

 and longitudinal sections. The development 

 of Monoxenia commences with the egg re- 

 peatedly dividing into many parts (C, D, JE). 

 This process, which is common throughout the 

 animal kingdom and is called egg-segmenta- 

 tion, in this case proceeds so simply and 

 regularly that it ends in the production of a 

 hollow sphere enclosed by a single layer of 

 cells (G). Each cell sends out a long cilia or 

 whip -like process (F) by means of which the 

 larva turns about and swims in the body-fluid 

 of the parent polyp. One -half of the sphere 

 now becomes infolded into the other half (H), 

 and forms what is called a gastrula (/, K). The 

 term gastrula has taken a great place in zoology 

 in recent years, since the Russian naturalist, 



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

 egg, passed through such a stage. Haeckel, generalising 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- 

 complete, 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 



Monoxenia darwini (highly magnified). 



