128 CCELENTERATA OS STINGING-ANIMALS. 



food-particles are not changed before absorption, the digestion 

 is intracellular. Some of the endoderm cells have muscular 

 roots lying, like those of the ectoderm cells, on the middle 

 lamina ; a few cells near the mouth and base are described 

 as glandular, and the presence of a few stinging-cells is also 

 alleged. 



The " middle lamina," representing the " mesogloea," is 

 a thin structureless plate, on each side of which lie the 

 muscular roots of ectodermic and endodermic cells. 



It is historically interesting to notice the important step 

 which was made when, in 1849, Huxley definitely compared 

 the outer and inner layers of the Coelenterata with the 

 epiblast and hypoblast which embryologists were beginning to 

 demonstrate in the development of higher animals. Not 

 long afterwards AUman applied to the two layers of Hydroids 

 the terms ectoderm and endoderm, which we have often used. 



The division of labour among the cells of Hydra is not 

 very strict, but already the essential characteristics of ecto- 

 derm and endoderm are evident. We may summarise these 

 as follows, comparing them with the characteristics of 

 epiblast and hypoblast in higher animals : — 



The Reproductive Organs. — {a) From nests of dividing 

 interstitial cells several clumps of spermatozoa or male ele- 

 ments are formed. They burst out at intervals through the 

 swollen ectoderm. 



{b) From a nest of interstitial cells, a single ovum is formed. 

 In rare cases there are two. The ovum is at first amoeboid 

 and transparent, it seems to grow at the expense of neigh- 

 bour cells. It becomes spherical, pigmented, and large, and 

 forms a prominent bulging on the ectoderm. 



