138 PHYLUM CELENTERA. 
primary opening of this cavity becomes the mouth of the adult, 
but in the more specialised types there ts an (ectodermic) oral 
invagination, which forms a gullet-tube or stomodeum. 
Between the ectoderm and endoderm of the body wall there 
7s a supporting layer, or mesoglea, often of jelly-like con- 
sistency, In Ctenophora, however, a more definite mesoderm 
zs established at an early stage in development. In the 
simplest cases the mesoglea is a secretion quite devoid of cells, 
but secondary cells may migrate into tt from the endoderm. 
Stinging cells of varying complexity are almost always present, 
but in most of the Ctenophora their place ts taken by adhesive 
cells, 
The Celentera exhibit two types of structure—polypotd 
and medusoid—which recur in modified forms throughout the 
group, and may be both present in the course of one life 
history, when they illustrate the phenomenon of alternation of 
generations, or metagenesis. The more primitive type is the 
sessile tubular polyp, which, at its simplest, may be com- 
pared toa gastrula fixed by one end, and furnished with a 
crown of tentacles round the central aperture of the other pole. 
The other derived form, which has. become specialised in 
various directions, 1s the active medusoid or jelly-fish type. 
In several divisions the formation of a calcareous “ skeleton” 
by the polypoid type results in the production of “corals.” 
Multiplication by budding ts common, and often results in the 
JSormation of colonies, some of which show considerable adivt- 
sion of labour. 
The preservation of the primary axts, the absence of true 
mesoderm and of a ceelom, are often said to distinguish 
Calentera and Sponges from the other Metazoa (Celomata), 
but the results of recent researches on the nature of the 
mesoderm seem to rob this distinction of part of its precision. 
GENERAL SURVEY 
The Ccelentera or ‘Stinging animals” include a large 
number of familiar and beautiful forms. The graceful 
zoophytes which fringe shells and stones, and the tiny 
transparent bells which float in the pools ; the sea-anemones 
which cluster in the nooks of the rocks, and the active jelly- 
fish which swim on the waves, are but different expressions 
