136 CCELENTERA. 



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

 period of quiescence lasting from one to two months. 



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 (Microkydra Ryderi) is known to liberate 

 free-swimming medusoids. This should be compared with the hydri- 

 form organism believed to be connected with the fresh-water Medusoid 

 Limnocodium found in the Victoria Regia tanks in the Botanic Gardens, 

 Regent's Park, London, and also in African lakes. 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 much 

 wanted. 



Second Type of Ccelentera. — A Medusoid. Class Hydro- 

 zoa. Sub-Class Hydromedusae or Craspedota. 



Hydra is too simple to be thoroughly typical of the 

 Hydrozoa. The class includes the hydroid colonies or zoo- 

 phytes, which may be compared to Hydra with many buds, 

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

 members of a hydroid colony, or (6) 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 (Tubularia). In some 

 of the hydroid colonies, notably the Millepores and Hydrac- 

 tinia, the polypes are very dissimilar to one another, and 

 have become specialised for the performance of different 

 functions. 



The medusoid type is like an inflated hydroid adapted 

 for swimming. It is bell-shaped, and down the middle of 

 the bell hangs a prolongation— the manubrium — which 

 terminates in the mouth. Around the margin of the bell 

 there is a little shelf, the velum or craspedon, which projects 

 inwards, and is furnished with muscle cells. The margin of 



