REPORT ON THE SIPHONOPHOR^E. 73 



This sail, or the vertical crest of the Velellidse, is the most characteristic part of their 

 trunk, and its gradual development is the primary cause of their peculiar amphithect 

 form. The sail is originally nothing other than a small fold of the exumbrella, or the soft 

 upper lamella of the disc. We may assume that the elevation of such a fold in any 

 ancestral Porpitidse was very useful as an aid to the locomotion of the pelagic animal 

 floating on the level of the sea by its horizontal disc. Natural selection, therefore, will 

 have increased the height of that crest, driven by the wind, and the immediate effect 

 must have been the transformation of the circular disc into an elliptical one, the sail 

 occupying the major axis of the ellipse (Rataria). Afterwards the soft and contractile 

 sail becomes supported by the development of an inner chitinous crest, arising from the 

 pneumatophore ( Velella), and finally the whole outline of the disc, and the arrangement 

 of its marginal parts, assumes the form of a parallelogram, and the sail is placed in its 

 diagonal axis (Armenista). A continuous series of intermediate transitional forms 

 conducts us from the elliptical Rataria (with sagittal sail), through different forms of 

 Velella, to the most specialised parallelogram-shaped Armenista (with diagonal veil). 

 The special form of the sail in the two latter genera depends upon that of the supporting 

 firm crest of the pneumatophore ; in Rataria, however, where the skeleton-crest is 

 wanting, its form differs greatly according to its varying state of contraction. 



Exumbrella. — That part of the trunk which includes the pneumatocyst, and cor- 

 responds to the invaginated exumbrella, is composed in the Velellidse, as in the other 

 Disconectge, of two parallel membranes connected by branched septa. The outer mem- 

 brane, or the permanent exumbrella (the uninvaginated part), is the pneumatocodon ; its 

 exodermal epithelium is armed with many cnidoblasts, and beyond it is placed a strong 

 muscular plate, composed of longitudinal or radial fibres. The inner membrane, or the 

 invaginated part of the exumbrella, is the pneumatosaccus ; its thinner exodermal 

 epithelium envelops like a complete sac the whole surface of the pneumatocyst, and 

 this is nothing more than the hardened chitinous cuticula secreted by the former. A 

 great number of septa connect both membranes, and between them occur the canals of 

 the exumbrella, more radially in the horizontal surface of the disc, more longitudinally 

 and parallel in the vertical surface of the sail (on both sides of it). These pallia! canals 

 open in the periphery of the horizontal disc into the marginal canal, and along the free 

 margin of the sail into a crescentic canal, running along the whole margin. 



Pneumatocyst. — The chitinous polythalamous float filled with air, which we call pneu- 

 matocyst (usually called the " inner shell "), always assumes the form of its surrounding 

 matrix, the pneumatosaccus. It is, therefore, in the Velellidaa elliptical or cmadrangular, 

 and very different from that of the Porpitidae and Discalidse, where it is always circular 

 and regularly octoradial. Since, however, the former have arisen originally from the 

 latter, their pneumatocyst also must be regarded as an amphithect or bilateral modifica- 

 tion of that circular float of the latter. Indeed, in all Velellidae there are traces, more or 



(ZOOL. CHALL. ESP. — PART LXXVIJ. — 1888.) Hhhll 10 



