l8o PHYLUM CCELENTERA 



The segmentation which follows is total and equal, and results in 

 the formation of a blastula (Fig. 89, 5). By inwandering, or by 

 division of the cells of the blastula, an internal endoderm is formed, 

 and this formation takes place on all sides. In a word, it is multipolar. 

 The segmentation cavity of the blastula is thus filled up, and the two 

 layers become differentiated from one another. 



The outer or ectodermic layer forms — (a) an external " chitinoid " 

 shell of several layers ; (b) an internal membrane, homogeneous, thin, 

 and elastic ; and (c) the future ectoderm of the adult. In Hydra fusca 

 the egg is separated from the parent before the shell is formed, and is 

 fastened by its gelatinous sheath to aquatic plants ; in H. viridis and 

 H. grisea the egg falls off after the outer shell has been formed. In 

 all species the separation from the parent appears to be followed by a 

 period of quiescence lasting from one to two months. It is probable 

 that this resting-stage is carried by wind and birds from one water basin 

 to another. 



Within the shell differentiation at length recommences, but it pro- 

 ceeds slowly. Interstitial cells arise in the ectoderm ; a middle 

 lamella is formed ; a gastric cavity begins to appear in the midst of the 

 endoderm. Thereafter the shell bursts, and development proceeds 

 more rapidly. The embryo elongates, acquires a mouth by rupture at 

 the distal (sometimes called vegetative) pole. The inner sheath is also 

 lost, and the young Hydra fixes itself and begins to live as its parent or 

 parents did. 



Forms like Hydra. — Even simpler than Hydra is Protohydra, 

 without tentacles, occurring both in the sea and in fresh water. An 

 American fresh-water form {Microhydra ryderi) is known to liberate free- 

 swimming medusoids {Limnocodium) which have been found in Europe, 

 e.g. in the Victoria Regia tanks in the Botanic Gardens, Regent's Park, 

 London. Another species, L. kawaii, has been found in the J angtszekiang 

 in China, 1000 miles from its mouth. A related form, Limnocnida, occurs 

 in Lakes Tanganyika and Victoria Nyanza, and in the river Niger. A 

 strange simple polyp — Polypodium — has been found as a parasite on 

 the eggs of sturgeons. Further details in regard to all these forms are 

 much wanted. 



Second Type of Ccelentera. — A Medusoid. 

 Class Hydrozoa 



Hydra is too simple to be thoroughly typical of the 

 Hydrozoa. The class includes the hydroid colonies or 

 zoophytes, which may be compared to Hydrce with many 

 buds, and also free medusoid forms, which may be (a) 

 liberated members of a hydroid colony, or {b) independent 

 organisms. Besides these there are complex colonies of 

 medusoid forms (Siphonophora). 



The hydroid type, except in minor details, usually 

 resembles Hydra. In some cases the tentacles are solid, 



