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



deeper, the palpons larger ; a pedicular canal (cp) passes from the bracteal canal to the 

 exumbrella (compare 84, p. 100, Taf. ii. figs. 17—22, y). 



The most remarkable larva, however, is the Physonula figured in PI. XIX. fig. 8 ; 

 it was very well preserved in a preparation in the Challenger collection, taken at Station 

 325, in the South Atlantic (March 2, 1876; lat. 36° 44' S., long. 46° 16' W.), in the 

 tow-net which had been down to a depth of 2650 fathoms. The nectosome was com- 

 posed of an apical pyriform pneumatophore (p) and two opposite nectophores (similar 

 to Dicymba, PI. XVIII. fig. 1). The siphosome consisted of a large spindle-shaped 

 siphon (s), with a simple cylindrical tentacle (t), and a basal corona of about a dozen 

 slender palpons (q), each provided with a thin and long palpacle (r). The trunk of 

 the nectosome in this monogastric larva was apparently the thin pedicle of the siphon 

 itself (am) ; its apex bore the pneumatophore, as the modified original umbrella of 

 the primary medusome. 



Synopsis of the Genera of Discolabidai. 



Nectosome biserial, with two opposite rows of nectophores, .... 55. Physophora. 



Nectosome quadriserial, with four cruciate rows of nectophores, . . . .56. Discolabe. 



Nectosome multiserial, with several continuous spiral rows of nectophores, . . 57. Stejihanospira. 



Genus 55. Physophora, 1 Forskal, 1775. 



Physophora, Forskal, 11, Descript., &c, p. 119. 



Definition. — Discolabidse with a biserial nectosome, composed of two opposite rows 

 of nectophores. (Siphosome with a double corona of palpons.) 



The genus Physophora, as one of the oldest known Siphonophorse, was founded in 

 1775 by Forskal, who distinguished three species of it (11, p. 119). The first of these, 

 Physophora hydrostatica, has been retained by later authors as the true t3^pe of the 

 genus, whilst the two others belong to different genera, Athorybia (rosacea) and Rhizo- 

 p>hysa (filiformis). Peron and Lesueur figured, in 1807, an Atlantic species under the 

 name Physophora myzonema (14, pi. xxix. fig. 4). Quoy and Gaimard published, in 1833, 

 an incomplete description of four species of Physophora ; they seem to belong to four 

 different genera (Physophora australis, Discolabe discoidea, Circalia alba, Sphyrophysa 

 intermedia). Lesson distinguished no less than eight species (3, p. 503). The first 

 accurate anatomical description of the typical Mediterranean Physophora liydrostatica 

 was given in 1853 by Kolliker (4), and Vogt (6), afterwards completed by Leuckart (8), 

 Huxley (9), Gegenbaur (10), and Clans (34, 74). The ontogeny of Physophora, and its 

 peculiar metamorphosis, were described by myself in 1869, as observed in a new splendid 

 Atlantic form, which I called Physophora magnified (84, Taf. i.-v.). A very careful 



1 Physophora = Bearing a vesicle, <pi/7o(foj«. 



