140 CGELENTERA. 



mode ; it occurs only in Coelentera, and in those which have no free 

 swimming blastosphere stage. Perhaps, as Brauer suggests, it was origi- 

 nally characteristic of free swimming spherical blastospheres, which 

 rotated in any direction. But when the blastosphere is oval, and swims 

 with one pole always forwards, the food particles are swept towards the 

 posterior pole. It is therefore advantageous that the formation of the 

 endoderm should occur there, either by immigration of cells, or by gas- 

 trular invagination (Korschelt and Heider). 



Forms like Hydra. 



Even simpler than Hydra is Protohydra, a form without 

 tentacles, occurring both in the sea and in fresh water. A 

 similar fresh water form has been described under the name 

 of Microhydra, and a strange simple polype Polypodium 

 whose history is incompletely known, has been found as 

 a parasite on the eggs of sturgeons. But further details in 

 regard to all these forms are much wanted. 



Second Type of CCELENTERA A MEDUSOID. Class HYDRO-' 

 ZOA. Sub-Class Hydromedusse 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 (^) independent organisms. 

 Besides these there are complex colonies of medusoid forms 

 (Siphonophorae). 



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 the bell also bears tentacles, usually hollow, and abun- 

 dantly furnished with stinging cells (Fig. 38, 3). 



