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



in PI. XXVI. fig. 1 ; it had a length of 4 mm. in the expanded, 2 mm. in the contracted 

 state, and was very similar to the youngest larvse figured by Huxley (9, pi. x. fig. l). 

 This young larva is a simple medusome, composed of an ovate pneumatophore (the 

 transformed umbrella) and a spindle-shaped siphon with a distal mouth (the manubrium 

 of the original Medusa) ; from the vesicular pedicle of the siphon, which connects it with 

 the base of the pneumatophore, arises a single long tentacle ; this is a simple cylindrical 

 filament, beset on the dorsal side with a series of reniform cnidonodes, of the same structure 

 as in the adult Physalidse. The ovate pneumatosaccus (fig. 1 , pf) exhibits an apical 

 stigma (])o), or a simple pore on the proximal pole of the longitudinal axis ; this is the 

 permanent opening of the original invagination. The cavity between the outer and inner 

 walls of the pneumatophore (closed above around the stigma) is below in open com- 

 munication with that of the pedicle, and by this with the cavities of the siphon as well 

 as the tentacle. The inside of the siphon is covered with numerous black hepatic villi 

 (sv) ; the margin of the mouth (expanded in fig. 1 as a scmare suctorial disc) armed with a 

 series of cnidocysts (ss). 



A second stage of Cystonula, 6 mm. in length, which I found recently in a prepar- 

 ation in the Challenger collection, and could not figure in PI. XXVI. (already printed), 

 is intermediate between figs. 1 and 2 of that Plate. It differed from the youngest stage 

 (fig. 1) in the production of a pair of buds from the ventral side of the dilated siphon- 

 pedicle (a), opposed to the dorsal tentacle (t). The posterior bud of this pair develops 

 into the first secondary siphon, the anterior into the first palpon (or basal sac) with its 

 tentacle. The dilated cavity between float [pf) and stomach (sv), from which the buds 

 arise, becomes now much larger and corresponds to the common stem or trunk («). 



The third stage of Cystonula observed is figured in PL XXVI. fig. 2 ; its length 

 in the expanded state was 8 mm. It is similar to the second form figured by Huxley, 

 10 mm. in length (9, pi. x. fig. 2). The slender pyriform trunk, with strongly inclined axis, 

 encloses in its proximal (anterior) half the pneumatosac, filled with air (pf), and bears on 

 its distal (posterior) end the protosiphon (the primary siphon or the manubrium of the 

 original medusome, with its tentacle). This is separated by a wide interval from the 

 central group, composed of three subequal cormidia which are attached to the middle 

 third of the ventral median line of the vesicular trunk ; each cormidium is composed of 

 two sterile persons arising from a common stem, a siphon (with hepatic villi, sv, and a 

 terminal mouth), and a mouthless palpon (or basal ampulla, with a long tentacle). 

 Regarded from the standpoint of our Medusome Theory, this Cystonula is a primary 

 medusome, which has produced by budding from its ventral side three pairs of 

 secondary medusomes ; these arise from the middle third of the trunk, or from that 

 portion which is the enlarged pedicle of the protosiphon (fig. 1, a). The growing 

 pneumatophore, which originally occupied the apical part of the primary larva, extends 

 now more and more over its dorsal side, descending in a basal direction. The longi- 



