38 Invertebrata. 



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 tenta- 

 cles, 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 bodies (p. 25), 

 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 be- 

 coming specialised and detached form in turn medu- 

 soids. There is thus an alternation of generations, the 

 progeny of the egg resembling, not the immediate egg- 

 producer, but the form which preceded this. 



Divisions of Hydroida. Of fixed Hydroids there 

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

 2. compound colonies whose stalks, polypites, and 

 medusoids are naked (fig. 2 1 ) ; 3. compound colonies 

 whose stalks and polypites are covered with a horny 

 casing 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 ccenosarc 

 extends, supporting both the nutritive and the repro- 

 ductive polypites. Some possess in addition a sac 

 filled with air which acts as a float and aids the 



