THE CCELENTERATA 159 



surrounding a central cavity. Ultimately a hollow sphere of cells 

 is produced, the blastula, in which is a large central space, the 

 blastoccel or segmentation cavity. The blastoccel is next filled by 

 cells derived from the periphery in two ways. Some of the outside 

 cells divide tangentially, and the more centrally situated of the two 

 daughter cells pass inwards, a process called multipolar delamination, 

 because it occurs at many points ; others of the cells from different 

 parts migrate into the centre, and this is termed multipolar im- 

 migration. Finally, as a result of these two processes, the sphere 

 becomes solid. The ingoing cells carry with them practically all 

 the yolk spheres, with the result that the embryo comes to 

 consist of an outer layer of more clear cells, the ectoderm, sur- 

 rounding an inner solid mass of granular cells, the entoderm, and 

 in this way the diploblastic condition is attained. This secretes 

 around itself a double-walled sheath, the outer layer being horny 

 and covered with spinous projections, and the inner, membranous. 

 All these changes take place while the egg is still attached to the 

 parent, but now it drops off and falls to the bottom of the pond. 

 After a period of rest the entoderm cells become arranged to form 

 a layer around a central cavity, the enteron or gastral cavity, and 

 then the capsule breaks, allowing the animal to creep out. At first 

 it has no tentacles nor mouth, but these soon appear much in the 

 same way as they do in a bud, and so we have produced a new 

 Hydra. 



This, then, completes the life history of a very simple Metazoon, 

 and although a primitive and, perhaps, even a degenerate form, it 

 serves as a type of the Phylum Ccelenterata, the majority of whose 

 members are, however, more complex. In some ways it marks a 

 very important step in advance of the Protozoa, the first stage in the 

 direction of that enormous complexity of cellular structure that 

 marks the higher animal. 



A Compound Coelenterate Obelia. 



We now pass on to consider a more highly-organised 

 member of the same Phylum, namely, Obelia. The genus Obelia is 

 a very common one around our shores, and, confined to the sea, it 

 is to be found just below low- water mark, sometimes, indeed, 

 exposed at spring tides, growing on stones, wooden piles and other 

 submerged objects. 0. geniculata, perhaps its most common 

 species, is frequently encountered as a pale greyish-brown moss-like 

 growth forming large conspicuous patches on the long fronds of the 

 oar weed (Laminaria). Its plant-like appearance is common among 

 the Hydrozoa, the class of the Ccelenterata to which it belongs, and, 

 in consequence, these animals have long been known to naturalists 



