10 SUEFACE FAUNA OF THE GULF STREAM. 



with that of a young Yelella, and differs radically from that given by 

 Pagenstecher for Rataria (as young Velella ?), in which the pneumatocyst' 

 clearly shows the eight primary divisions so characteristic of Porpita with the 

 central opening of the disk. Huxle}' also observed that in young Velellae 

 the hepatic mass does not exist, and is only gradually developed under the 

 pneumatocyst, and that the canals which cross it are mere subdivisions of 

 the somatic cavity produced by the lobes of this organ and their mutual 

 anastomoses. My observations on Velella lead me to agree with Kolliker re- 

 garding the mode of junction of the liver canals and of the canals on the 

 free edge of the mantle and on the upper part of the float. The dendritic- 

 like structure of the canals of the crest have also been noticed by Huxley in 

 a young Velella measuring somewhat less than half an inch in length. The 

 circulation within the canals was most active, and wholly due to ciliary action. 

 The figures given by Huxley (page 126, Oceanic Hydrozoa) of the peculiar 

 corrugations and lobes of the lower surface of the float, being taken from 

 alcoholic specimens, are not quite satisfactory representations of their appear- 

 ance in fresh specimens. 



As has already been suggested by Kolliker, Agassiz, and McCrady, the re- 

 lationship of the Velellidae and PorpitidiB, as well as the Physalida?,* to the 

 Tubularian Hydroids is very great. If we compare this group as a whole 

 to the other Siphonophores, the absence of " Deckstticke " and of swimming 

 bells seems to distinguish them from all the other Siphonophores except 

 Rhizophyza, which perhaps is only a representative of the embryonic stage 

 of the Physalida^, and does not belong into the close association with the 

 other Siphonophores, with which it has usually been placed. What is 

 known of these different families seems to indicate a far closer relationship 

 with the Tubularian Hydroids, such as Hydractinia, which may perhaps be 

 the closest ally of the genera named above, and in which the chitinous exten- 



* The chambers of the crest of Pbysalia can be considered as a sort of girder wbicli stift'ens the whole 

 float, and to a certain extent takes the place of the chitinous crest of Velella. The structure of the crest 

 is seen in section to be a broadly rectangular triangular cell, subdivided by horizontal bars to form the 

 smaller trapezoidal cells of a second, third, fourth, and fifth story, adjoining triangles being again con- 

 nected loni^itudinally by similar bars. The float of Physalia remains in fact in the embryonic stage in 

 which we find the sail or crest of Velella and young Porpita. In the latter the crest gradually disappears 

 to cover the upper part of the float ; in the former it continues through to the mature stage, being sup- 

 ported by a chitinous vertical projection from the float, which is absent in younger stages, while in 

 Physalia we have only the mantle, if I may so call it, of the crest left, the pneumatophore not secreting a 

 chitinous float or any structure homologous to the circular chitinous float of Velelhi or the chitinous disk 

 of Porpita. The presence of a net-work of canals at the base of the float of Physalia, similar to that of 

 the upper part of the float of Velella Porpita, has been made probable by some observations of Quatre- 

 fages. (Ann. Scien. Nat. 1853). 



