38 Inverted rata. 



Unlike as this jellyfish may appear to be to Hydra, 

 it is in reality a polypite modified by a widening and 

 thickening of the body wall at the base of the tentacles, 

 and an elongation of the mouth into a central stalk. 

 These free-swimming jellyfishes are thus not distinct 

 animals, but only detached parts of hydroid colonies, 

 and the eggs produced by them first give origin to 

 small ciliated, infusorium-like but multicellular bodies, 

 called planulae (fig. 19, A) which after a short period of 

 freedom settle on stones or shells at the sea-bottom, 

 and develop into primary polypites and the common 

 stem of a new colony, from which some buds becom- 

 ing specialised and detached form, in turn, medusoids. 

 This life-history is an example of an alternation of gene- 

 rations, the progeny of the egg resembling, not the im- 

 mediate egg-producer, but the form which preceded this. 



Divisions of Hydromedusae. Of fixed Hydroids 

 there are three orders : i. simple forms, such as Hydra ; 

 2. compound colonies whose stalks, polypites, and medu- 

 soids are naked (fig. 21) ; 3. compound colonies whose 

 stalks and polypites are covered with a horny casing or 

 perisarc, as in the true sea-firs (Sertularia, fig. 20). 



Sub-Class 2 : Siphonophora. Floating on the sea 

 there are frequently found colonies of hydroid poly- 

 pites, not unlike those of the sea-firs in structure, but 

 whose common stem instead of being rooted, swims 

 by means of enlarged and altered polypites, whose 

 stomachs are undeveloped, and whose bodies are 

 dilated into swimming bells. From these, the coenosarc 

 extends, supporting the nutritive, protective, and re- 

 productive polypites. Some possess in addition a sac 

 filled with air which acts as a float and aids the 



