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



4 to 5 mm. in diameter, decreases also towards the two ends of the spiral, according to that 

 of the inserted cormidia. Each single cormidium (PL XX. fig. 16) is composed of the 

 following parts, which succeed one another in the distal direction, beginning from the 

 uppermost part of the facette : — (1) a simple palpacle (r) ; (2) a large palpon (j)) ; (3) a 

 female gonodendron (gf) ; (4) a male gonostyle (gh) ; (5) a tentacle (t) ; and (6) a 

 siphon (s). Since the tentacle is only an organ of the siphon, and the palpacle an 

 organ of the palpon, each cormidium may be explained as originally an association of 

 four medusomes, two fertile and two sterile ; the umbrellas belonging to the latter may have 

 been originally the nectophores, widely dislocated and separated. The common base of the 

 trunk from which these parts of each cormidium arise, corresponds to a node or a lateral 

 branch of those Siphonanthai which have prolonged stems with large internodes and 

 widely distant ordinate cormidia. The muscular frame which surrounds each polygonal 

 facette corresponds to the muscle-group of the internode. The facette itself, which 

 becomes visible after the detachment of the palpons, above the gonodendra, is the base of 

 their insertion, with the palponal canal in the centre (figs. 9-13, cq). 



Siphons (PI. XIX. fig. 1 ; PI. XX. figs. 13, 16, s). — The single siphon of each cormidium 

 is inserted into its distal side or the inferior part of the node of the trunk; this is the inner- 

 most part in the subhorizontally coiled-up trunk, on the concave inside of the spiral bladder. 

 When the siphons are detached from their pedicles, and the trunk seen from below (PI. XX. 

 fig. 12), the conical papilliform pedicles remaining connected with the trunk form a 

 regular dexiotropic spiral line on its basal face (fig. 12, sp). The siphon proper, besides 

 the pedicle, is spindle-shaped or ovate, and has in the dilated state a length of 20 or 

 30 mm. or more, in the contracted state scarcely 4 to 8 mm. Its basigaster is ovate, 

 large, the thick- walled exoderm full of small cnidocysts. The dilated stomach is rather 

 thin-walled, inside covered with vacuolated villi, without coloured hepatic stripes. The 

 contractile proboscis is very dilatable, with eight strong longitudinal muscle-bands. The 

 distal mouth may be expanded in the form of a large suctorial disc, and is sometimes 

 circular, at other times slightly octolobate (figs. 13, 16, so). 



Tentacles (PI. XIX. fig. 1 ; PL XX. figs. 13-16, t). — The long capturing filament, 

 which arises from the pedicle of each siphon, near the attachment of its basigaster, bears 

 a series of very numerous large tentilla. Each fully-developed tentillum exhibits the 

 peculiar well-known structure of Physophora, and is composed of two parts, a long pedicle, 

 the distal portion of which is inflated, and a large pyriform cnidosac (fig. 14). This latter 

 contains, included in a double involucre, a very large, spirally coiled up and reflected pink 

 cnidoband. The two lateral groups of large ensiform cnidocysts, which were originally 

 placed on the proximal end of the cnidoband, lie in the fully-developed tentillum 

 on its distal end ; this remarkable inversion is effected by the peculiar development of 

 the involucre described above (p. 260). The proximal base of the involucre is con- 

 nected in the median line of its concave ventral side with the distal end of the reflected 



