4H2 PROFESSOR ALLMAN ON THE RELATIONS OF THE C03LENTERATA. 



The hydrocysts (g) of the siphonophore are plainly arrested polypites, in 

 which the mouth has never become developed. 



Again, the generative zooids (i) are exactly paralleled by those of the Hy- 



droida, and are, like them, referable to two 

 types, expressed in the Hydroida by the phane- 

 rocodonic and the adelocodonic gonophores, the 

 situation of the generative elements being pre- 

 cisely similar in the two orders ; while the necto- 

 calices or locomotor zooids (k, k) are essentially 

 hydroid medusa, with specially developed um- 

 brella, but with the manubrium suppressed, and 

 the somatic cavity reduced to the atrium, from 

 which spring radiating canals, which, exactly as 

 in the hydroid medusa, open round the margin 

 into a circular canal. 



The bracts or hydrophyleia (/) of the siphono- 

 phore are essentially csecal offsets from the com- 

 mon canal of the ccenosarc, but with the ecto- 

 derm 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 dif- 

 ferent functional destination, diverge widely from 

 these, and constitute an apparatus of protection 

 instead of locomotion. 



All these zooids are kept in union with one 

 another by a coenosarc (m, m), which, in the typical Siphonophora, is fili- 

 form, with an axial canal in free communication with the cavity of each 

 of its appended zooids, thus corresponding essentially with the filiform 

 tubular coenosarc of a hydroid colony ; while in the somewhat aberrant 

 forms with fusiform or discoidal coenosarc [Plnjsalidce, Velellidce), an ob- 

 vious comparison is suggested with the appressed expanded coenosarc of 

 Hildractinia. 



From the hydroid coenosarc, indeed, that of the Siphonophora mainly 

 differs in the absence 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 contrac- 

 tions of the nectocalices. In the siphonophorous section, P1iysophorida>, 

 the proximal extremity of the coenosarc, instead of forming, as in the 

 Hydroida, a hydrorhiza for fixation, is modified by an inversion of its 



Fig. 7. — Diagram of a Siphonophore. 

 r, Polypite ; /, tentacle springing from 

 proximal end of polypite ; f, branches 

 given off by the tentacle ; g, hydrocyst; h, 

 tentacle of hydrocyst ; i, generative zooid 

 representing the phanerocodonic gono- 

 phore of a hydroid ; k, k, nectocalices; I, 

 bract ; m, m, ccenosarc ; n, pneumatoeyst. 



