SECT. in. COMPOUND OCEANIC HYDROZOA. 87 



sought for amidst the luxuriant marine vegetation and 

 profusion of animal life which adorn these rocky pools, 

 otherwise they would escape notice; and even when 

 large enough to be conspicuous, the eye must be aided 

 in order to see the wonderful minuteness and delicacy 

 of their structure. The aquaria have furnished an op- 

 portunity to study their forms, habits, and the marvellous 

 circumstances of their lives and reproduction. 



The compound oceanic Hydrozoa are essentially the 

 same in structure as the compound fresh-water Hydrse. 

 They differ, however, from them in often having a 

 greater number of tentacles, and in being defended 

 by a firm and flexible horny coat ; notwithstanding 

 which they increase in size by budding from the base 

 of a single primary polype. The horny coat covers the 

 bud and grows with it; but as soon as the polype is 

 formed within it, the top of the bud opens and the 

 young polype protrudes itself, so that a separation is 

 effectually prevented ; and while the stem and branches 

 are being formed, and increase by the continual deve- 

 lopment of new buds, the communication between the 

 stomachs of the whole brood of polypes with that in the 

 parent stem is maintained by tubes from their bases 

 passing through the interior fleshy matter in the 

 branches. 



In short these marine Hydrozoa consist of a ramified 

 tube of sensitive animal matter, covered by an external 

 flexible and often jointed and horny coat or skeleton, 

 and they are fed by the activity of the tentacles and the 

 digestive powers of frequently some hundreds of hydra- 

 formed polypes, as in the Sertularia cupressina. The com- 

 mon produce of their food circulates as a fluid through 

 the tubular cavities, for the benefit of the whole commu- 

 nity, while the indigestible part is ejected from the mouth 

 of each individual. The stomach of each polype has a 

 more or less ciliated lining, containing cells with nutri- 



