196 



HOMOLOGIES. 



the adclocodonic gonophores, the situation of the generative elements being precisely similar in 

 the two orders. So also the nectocalices or locomotor zooids {k, k) are essentially hydroid medusae 

 with specially developed umbrella, but with the manubrium suppressed and the somatic cavity 

 reduced to the atrium, from which spring radiating canals which, exactly as in the hydroid 

 medusae, open round the margin into a cu'cular canal. 



The bracts or hydrophyleia (/) of the siphonophore are essentially cscal offsets from the 

 common canal of the coenosarc, but with the ectoderm greatly developed and modified, as in the 

 umbrella of a medusa, so as to fit them to become organs of protection for the other zooids 

 They have thus essentially the same morphological foundation as the nectocalices, but with a 

 different functional destination diverge widely from these, and constitute an a])paratus of protec- 

 tion instead of locomotion. 



All these zooids are kept in union with one another by a coenosarc {m, m), which in all the 

 typical SiPHONOPHORA is filiform, with an axial canal in free communication with the cavity of 

 each of its appended zooids, thus corresponding essentially with the filiform tubular ccenosarc of a 

 hydroid colony ; while in the somewhat aberrant forms with fusiform or discoidal coenosarc 

 {PliysalidcB, VeleUidce) an obvious comparison is suggested with the appressed, expanded coenosarc 

 of Hydractinia. 



From the hydroid ccenosarc, indeed, that of the Siphonophora mainly differs in the absence 



Fig. 69. 



Fig. 70. 



Diagramatic longitudinal section of a Discophorous 

 Medusa. 



a. Radiating canal; h, manubrium; J', somatic cavity; 

 d, generative pouches ; o, o, o, o, pillar-like offsets from the 

 oral side of the umbrella ; z, tentacula-like appendages 

 of the inner surface of the somatic cavity. 



Diagramatic transverse section of a 

 Discophorous Medusa, 

 a, a, a. Radiating canals ; 6, manu- 

 brium; rf, rf, generative pouches; o, o, um- 

 brello-manubrial pillars. 



of an external chitinous sheath and in its free mode of existence, the siphonophore dwelling at 

 large in the open sea, through which, in the great majority of the order, it is propelled by the 

 contractions of the nectocalices. In the section PhysophoridcB the proximal extremity of the 

 ca3nosarc, instead of forming, as in the Hydroida, a hydrorhiza for fixation, is modified by an 

 inversion of its walls, so as to constitute an air -filled chamber or pneumatocyst («), which acts as 

 a float.^ 



^ In the above comparison of the Siphgnophoka with the Hyduoida, I have adopted generally 

 the views of Huxley, who was the first to bring out in a complete form the liomological relations 



