REPORT ON THE SIPHONOPHOR^E. 77 



centradenia in the Velellidae is probably due to their pelagic habit of life, and the 

 development of the vertical sail as an excellent means of passive locomotion. The 

 Porpitidas, on the other hand, are probably inhabitants of different depths, and only 

 occasionally come to the surface ; their hydrostatic apparatus and the gas-secreting gland 

 are therefore more developed. 



Gonostyles. — The numerous polypites of the . subumbrella, which produce by budding 

 the medusiform gonophores, are in the Velellida± mouth-bearing siphons, as in the 

 Porpitidae, not mouthless palpons as in the Discalidae. They are, therefore, usually 

 called "smaller polypes," " sexual polypites," or "peripheral siphons" (shortly "peri- 

 siphons "). They occupy usually, densely crowded in great numbers, a broad gono- 

 stylar zone, and often the whole space of the subumbrella between the central 

 siphon and the submarginal corona of tentacles. But at first their number seems to be 

 restricted to eight or sixteen ; at least this is the case in Rataria cristata (PI. XLIV. 

 fig. 2), and also in some similar young larvae (Ratarula) of Velella. Their form and 

 structure are the same as in the Porpitidse, already described above (p. 36). 



Tentacles. — The corona of submarginal tentacles is in the Velellidas far less developed 

 than in the Porpitidae ; their structure, too, is simpler than in the latter. The corona is 

 simple, composed of a single series of filaments in Rataria and Velella ; it is double or 

 multiple, and composed of two or three (seldom more) series in Armenista. Their 

 number is probably originally eight, and this occurs in some Ratarula-larvaa ; but there 

 are other similar larvae in which the corona bears a variable number of tentacles irregu- 

 larly disposed. Some very small and young larvae of the Ratarula-form exhibit only 

 two tentacles, at opposite poles of the major axis of the ellipse, and corresponding to 

 the two primary stigmata of the pneumatocyst. 1 The only Rataria which 1 have 

 observed in the adult state (with gonophores) possessed sixteen tentacles, rather regu- 

 larly disposed along the limb (PI. XLIV. figs. 1, 2). In Velella this number is soon 

 increased, and amounts in the simple series usually to fifty to eighty, often more than a 

 hundred. In A rmenista there are usually some huudreds, or sometimes thousands ; the 

 smaller tentacles of the outer (or distal) series alternating with larger filaments of the 

 inner (or proximal) series. 



Form of the Tentacles. — The general structure of the tentacles in the Velellidae is the 

 same as in the Porpitidae, but their form is much simpler, and the three rows of stalked 

 cnidospheres, which are characteristic of the tentacles of the latter famdy, are wanting. 

 The submarginal filaments of all Velellidae are simple cylindrical tubes, sometimes 

 slightly compressed, usually gradually tapering towards the rounded distal end, seldom 

 somewhat club-shaped ; in very young larvae they are conical and pointed. Their arma- 

 ture with cnidoblasts is very different from that of the two other families. Usually each 

 tentacle bears only two lateral ribands of cnidoblasts, sometimes four (two stronger 



1 Compare Bedot, 60, 1884. 



