94 THE VOYAGE OF H.M.S. CHALLENGER. 



uppermost part usually includes a smaller or larger oil globule, the oleocyst (co). The 

 physiological function of the somatocyst may be hydrostatic (as a float) and nutritive 

 (as an accumulation of nutritive, strongly refracting albuminous globules). Its morpho- 

 logical nature is explained by the medusome-theory which compares it with the apical 

 canal or original peduncular canal of a Medusa-person. 



Trunk or Ccenosarc. — The common stem in all polygastric Calyconectse is a long 

 cylindrical and highly contractile tube, very long and thin in the expanded state, short 

 and thick in the contracted state, when it is retracted into the hydroecium. The cormidia 

 are always ordinate, arranged in a single series on the ventral side of the articulated stem ; 

 they are separated by free naked intern odes of equal length. Very rarely (in Polyphyes) 

 the cormidia begin to be scattered. The number of the cormidia is in the smaller cormi 

 ten to twenty (rarely less), usually forty to eighty or more, sometimes several hundreds. 

 In the largest species (mainly of Praya) the expanded stem attains a length of more than 

 a metre. The structure of the stem-wall is that usually found in the Siphonanthse ; the 

 tubular fulcrum (or structureless supporting plate) is invested on its inner side by a thin 

 layer of entodermal circular muscles, on the outside by a strong layer of exodermal longi- 

 tudinal muscles ; these are arranged, as usual, in parallel bundles along the lamellar 

 radial folds of the fulcrum. 



Cormidia. — The aggregation of different medusoid persons, by which the cormus of 

 the Calyconectse is formed, follows certain simple and regular laws, but is different in 

 the two kinds of cormidia, which we distinguish as Eudoxomes and Ersseomes. The 

 cormidia of the great majority of Calyconectse are Eudoxomes, or in the free inde- 

 pendent state " Eudoxise " or " Diphyozooids " ; each Eudoxome is a twin-group, composed 

 of two medusoid persons, a fertile and a sterile medusome. The sterile medusome is 

 composed of a bract, a siphon, and a tentacle. The fertile medusome is represented 

 originally by a single medusiform gonophore, but afterwards this is often replaced by a 

 cluster of several gonophores. 



The Ersseomes (or the monogastric generation of Lilyopsis and Diphyopsis) differ 

 from the Eudoxomes in the fact that the primary gonophore loses its sexual manubrium, 

 and is converted into a so-called " special nectophore " ; its sexual function is replaced 

 by a secondary gonophore. The Ersseome, therefore, is composed of three medusoidal 

 persons, a sterile medusome (bract, siphon, and tentacle), a sterile nectophore, and a 

 fertile gonophore. Afterwards the latter is often replaced by a cluster of several 

 accessory gonophores. 



The sessile gonophores of the Eudoxomes and Ersseomes attain sexual maturity, whilst 

 attached to the trunk, in Mitrophyes and Cymbonectes among the Monophyidse, Praya 

 and Gaholaria among the Diphyidoe, probably in all Desmophyidse and Polyphyidse. 

 This is not the case in the majority of Monophyidse and Diphyidse. Here the cormidia 

 become detached from the common stem before reaching maturity, and swim freely 



