TYPES OF CCELENTERA A MEDUSOID. 149 



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 ^sin 

 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. A fresh-water Medusoid Limnocodium was 

 found in the Victoria Regia tanks in the Botanic Gardens, Regent's 

 Park, London. Its native habitat is unknown. Another species, 

 L. kawaii, has been found in the Jantszekiang in China, 1000 miles 

 from its mouth. A related form, Limnocnida, occurs in Lakes Tangan- 

 yika and Victoria Nyanza, and in the river Niger. A strange simple 

 polype Polypodium has been found as a parasite on the eggs of 

 sturgeons. Further details in regard to all these forms are mucn 

 wanted. 



Second Type ^/"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 Hydra with many 

 buds, and also free medusoid form c , 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, 

 instead of "hollow as in Hydra^ and they may be arranged in 

 two circles, an outer and an inner (e.g. Tubularia). In 

 some of the hydroid colonies, notably the Millepores and 

 Hydractinia, the polyps are very dissimilar to one another, 

 and have become specialised for the performance of different 

 functions. 



