350 THE VOYAGE OF H.M.S. CHALLENGER. 



apex of the float (or the anterior top) exhibits a beak-shaped, very mobile apophysis, the 

 apical rostrum ; and at the upper side of its base is the stigma, or the air-pore (po). 



Cormidia-. — The posterior half of the ventral side of the trunk bears a continuous series 

 of densely clustered cormidia, of the same shape as in the younger and smaller forms 

 of Caravella maxima. Each fully developed cormidium (fig. 6) bears on a common 

 short cylindrical pedicle (ap) four different organs, viz., (1) a blue siphon with black 

 hepatic villi (sv) and a distal mouth (so); (2) a long blue or red tentacle beset with a series 

 of reniform cnidonodes (t) ; (3) a slenderly spindle-shaped, light greenish basal ampulla 

 (to) arising from its base; and (4) a clustered, yellowish or reddish, monostylic gono- 

 dendron of the usual composition (p. 313). The structure of all these parts is the same 

 as in the other Physalidee, as described above (pp. 345-347). 



A variable number of main tentacles (usually six to eight) are far larger than the 

 others, besides the young and undeveloped ones. The primary basal siphon (or the 

 protosiphon, fig. 4, su), at the distal end of the trunk, is of the same form and size as 

 the secondary siphons (in the ventral side), but is sterile and bears no gonodendron. 

 Between it and the lowermost (oldest) secondary siphon is a group of small pyriform 

 palpons (without tentacles). This group fills the interval (or the basal internode), 

 which is free and naked in Alophota (PI. XXVI. fig. 3). 



Genus 75a. Physalia, 1 Lamarck, 1816. 

 Physalia, Lamarck, Hist. nat. des anim. sans vert., torn. ii. p. 478. 



Definition. — Physalidse with a polythalamous crest on the dorsal side of the large 

 vesicular pneumatophore. Siphosome with a single large main tentacle. 



The genus Physalia and the following Caravella make up together the subfamily 

 Caravellidse, differing from the preceding subfamily Arethusidae in the possession of that 

 peculiar dorsal crest of the pneumatophore, which is divided by numerous vertical 

 transverse septa into a series of air-filled triangular chambers. All Physabdas belonging 

 to the Caravellidae attain a far larger size and bear a far greater number of organs in their 

 loose cormidia than the preceding Arethusidae. The genus Physalia, in the stricter 

 definition here offered, comprises the greater number of species hitherto described. It 

 bears only a single large main tentacle, much longer and thicker than the numerous 

 accessory tentacles, and differs by this constant character from the following Caravella, 

 provided constantly with a greater number of subequal strong main tentacles (usually ten 

 to twenty or more). Physalia bears therefore the same relation to Caravella as the 

 crestless Alophota does to Arethusa. 



The accurate distinction and sharp definition of the numerous species of Physalia which 

 have been described is a very difficult task, owing partly to the numerous transitional 



1 Phrjsalia = Sea-bladder, tpvvct, SlKih. 



