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



canal. All other Diphyidse possess a somatocyst in the first or apical nectophore ; it 

 must be regarded as the uppermost part of the original common stem, overgrown and 

 enclosed by jelly-substance of the first nectophore. The somatocyst is usually rather 

 large, spindle-shaped, or ovate, sometimes more cylindriaal, at other times more ovate. 

 Usually it ascends from the apex of the hydrcecium ; but in Abyla it descends along its 

 ventral side. The structure is the same as in the other Calyconectse (compare above, 

 p. 93). 



Siphosome. — The tubular trunk or common stem of the Diphyidse is very contractile, 

 and beset at regular intervals with the cormidia, the number of which is very variable. 

 The stem is very long in the lower and older forms of the family, in the Prayidse and 

 Galeolaria, where it sometimes attains a length of more than one metre, and bears 

 more than one hundred cormidia. Their size and number are much smaller in the 

 specialised Abylidse, intermediate in the Diphyidse. The contracted stem may usually 

 be retracted more or less completely into the hydrcecium. The structure of the stem is 

 described above (p. 94). 



Cormidia. — The cormidia of the Diphyidse, or the Diphyozooids of Huxley (9, pp. 

 57-66), occur in two different principal forms, eudoxomes and ersseomes. The majority 

 of the genera possess eudoxomes ; each cormidium is composed of a sterile medusome 

 (bract with siphon and tentacle) and a fertile medusome (gonophore). The two genera 

 Lilyopsis and Diphyopsis possess ersseomes, a sterile special nectophore, as locomotive 

 person, being added to the euxodome. 



Bracts. — The bracts or hydrophyllia are of very different form and structure, 

 characteristic of the single genera and even of the three subfamilies. The bracts are 

 mitriform and rounded in the Prayidse, spathiform or conical in the Diphyopsidaa, 

 prismatic or polyhedral in the Abylicke. Besides, the form and place of the phyllocyst, 

 and the number, form, and course of the radial canals which arise from its base, 

 exhibit characteristic differences in the various genera. 



Siphon and Tentacle. — The form and structure of the polypites exhibit no 



important differences in the cormidia of the various Diphyidse. The structure, too, 



" of the tentacles is in general the same ; but the special form of the tentilla, and 



especially the composition of the cnidosacs and the arrangement of their different cnido- 



cysts, are subject to many specific variations. 



Eudoxiee. — The minority of the Diphyidse produce sessile eudoxomes, which maturate 

 whilst attached to the stem. This is the case in some of the Prayidse and in Galeolaria. 

 In all the other Diphyidse they become early detached from tiie stem, and maturate as 

 free Eudoxise (compare above, p. 101). 



Ontogeny. — On the development of the Diphyidse, compare above, pp. 100-102. 



