REPORT ON THE SIPHONOPHORjE. 343 



different buds, the siphons on one side, and the palpons and tentacles on the other side; 

 afterwards all the gonodendra lie on one side (right or left, figs. 3, 4). The more the trunk 

 becomes inflated by the hypertrophied float, the more it lies on one side of the body. On 

 the opposite side is developed the crest of the Caravellidse, acting the part of a sail, similar 

 to that of the Velellidae. As in these latter, the unilateral situation of the sail (right or 

 left) is accidental, and is not constant in the single species ; but usually in each species (as 

 with the eyes of the Pleuronectidse) the majority have the crest on the same side (compare 

 Chun, 83, p. 576). The largest siphons and palpons, and the main tentacles, lie on this side 

 (the lophopleura), while the gonodendra develop on the opposite side (the hypopleura). 

 Compare L. Agassiz, 36, p. 335. The different growth of the two antimeres (or body-halves) 

 is in some Physalise {e.g., Physalia utriculus, 77, Tab. xxxv. fig. 2) so striking, that the 

 median plane of the dorsal crest lies more horizontally than vertically, and the usual 

 ovate or pyriform shape of the float becomes triangular; the distance between the anterior 

 stigma (on the apical pole of the main axis) and the posterior protosiphon (on its basal 

 pole), seen from above, is in this case often scarcely half as great on the lee-side, or the 

 hypopleura (which bears the gonodendra), than on the windward-side or the lophopleura 

 (which bears the crest). It must be remembered, however, that the free edge of the 

 comb-like crest is always originally the dorsal median line of the asymmetrical trunk, 

 and the line in which the cormidia bud the ventral median line. The anterior stigma 

 marks constantly the apical pole of the longitudinal main axis, and the mouth of the 

 posterior protosiphon its basal pole. 



Crest of the Float. — The remarkable polythalamous comb-like crest of the 

 pneumatophore, which is usually regarded as the most striking peculiarity of this family, 

 is developed only in the larger Caravellidse (Physalia and Caravella) ; it is wanting in 

 the smaller Arethusidae, which, because of their much smaller size and simpler 

 form, have usually been overlooked (Alophota and Arethusa, PL XXVL). It is 

 wanting, also, in the younger larvae of the Caravellidae. The crest, therefore, 

 is a secondary organ, got by adaptation to the. sailing locomotion of the hydro- 

 static float. Eegarded from a morphological point of view, it is nothing more 

 than a simple longitudinal fold in the dorsal median hue of the trunk. It 

 becomes divided afterwards by a number of transverse septa into a series of 

 triangular air-chambers. These have often been compared with the chambers 

 in the polythalamous pneumatocyst of the Disconectse. But this comparison is only a 

 remote analogy, not a true homology. The morphological affinity which is suggested by 

 most authors between Physalidae and Velellidae does not exist at all. On the structure of 

 the crest and its relation to the float, compare Leuckart (81, p. 192), L. Agassiz (36, p. 335, 

 pi. xxxv.), and Chun (83, p. 576). The number of the primary chambers in the crest of 

 young Caravellidae is three or four, in the older six to eight or more. These become 

 divided by smaller transverse septa into secondary chambers, and these again by smaller 



