IX. OBELIA 



Obelia is a marine animal and a typical representative of a 

 considerable number of species of Coelenterate animals which 

 exhibit the phenomenon of alternation of generations, or meta- 

 genesis, as this process is often called in animals. In Obelia, the 

 life history includes a stage in which the organism consists of a 

 colony of sessile, asexual individuals which are attached to a com- 

 mon stalk. This stage alternates with the medusa, or jellyfish, 

 stage in which only independent, free swimming, sexual individuals 

 occur. Among the various groups of Coelenterates wide varia- 

 tions in life history are found, ranging from a condition, as in 

 Hydra, in which no medusa stage is known, to that in the true 

 Jellyfishes, in which no adult stage comparable to Hydra occurs. 

 Another noteworthy feature of many species of Coelenterates 

 is the development of colonies of Hydra-like individuals as in 

 Obelia, but with the formation of great amounts of secreted cal- 

 careous material between the individuals in the colony. Such 

 is the case in the common Corals, several species of which are 

 shown on page 53. 



A. Structure of the Asexual Stage of Obelia 



The general appearance of the asexual colonial stage of this 

 animal is superficially plant-like. It consists of a colony of at- 

 tached individuals, termed polyps, which are connected by a com- 

 mon stalk. The stalk is differentiated into two portions. There 

 is, first, the part known as the hydrorhiza which is attached to 

 some convenient solid object in the water, and, second, the 

 branches, or hydrocauli, which arise from the hydrorhiza and 

 bear the polyps at their tips. Both of these portions of the 

 stalk are composed of an outer, transparent, exoskeletal sheath, 

 the perisarc, which is non-living, and an inner, living portion, the 

 coenosarc, which is continuous with the living material of the 

 polyps. (W. f. 37, A : B. f. 9.) 



The polyps composing a typical, fully-developed colony are of 

 two kinds. There are, first, the nutritive polyps, or hydranths, 



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