REPORT ON THE SIPHONOPHORjE. 75 



axis. The ellipses are short and nearly circular in the young Rataria ; they become 

 longer and sometimes slightly quadrangular in Velella, and completely parallelogram- 

 shaped in Armenista. The number of these concentric ring-chambers is very variable in 

 the different species, two to eight only in the smallest species {Rataria), twenty to 

 thirty or more in the larger {Velella), and fifty to eighty or more in the largest forms 

 {Armenista). 



Pneumothyrse. — The concentric ring-chambers of the pneumatocyst are all in open 

 communication one with another. Each annular elliptical septum between two chambers 

 is pierced by two opposite openings or septal pores, one in the dorsal half, the other in 

 the ventral half of the disc. All these septal pores are placed in a straight line, cutting 

 the major axis of the disc at a very small angle, as was first shown by Kolliker. 1 There 

 are, therefore, not eight radial rows of pneumothyrse as in the ancestral Porpitidae, but 

 only two opposite radial rows. In some species of Velellidae, however, there seem to 

 exist two other opposite rows, placed in a transverse line near the frontal groove. 



Stigmata. — All Velellidae possess a relatively small number of superficial openings on 

 the ring-chambers, which lie at their apical or superior surface, and serve for the emission 

 of gas. The number of these stigmata is much smaller than in the Porpitidae. Usually 

 only every third or fourth ring possesses a pair of opposite stigmata ; sometimes rings 

 with and without stigmata alternate regularly ; rarely each ring possesses one pair. All 

 the gas-pores or stigmata discovered by Kolliker be in a straight line, just as do the 

 septal pores, and the line of the former is placed' between the fine of the latter and the 

 base of the vertical crest. Half the stigmata, therefore, lie in the ventral half of the disc 

 on the right side of the crest, and the other half in the dorsal half on the left side, or 

 vice versa (compare PI. XLIII. fig. 5, pe). The direction of the series of stigmata in all 

 Velellidae is already marked by the three first stigmata of their Katarula-larvae. These, 

 as well as the mature Rataria (PI. XLIV. fig. 8), possess only three stigmata, which are 

 placed in a line, cutting the sagittal axis of the horizontal disc at an acute angle ; one of 

 the three pores lies near the centre {po) ; the second on the left {pe), and the third on 

 the right side. All the following stigmata develop in the direction of that line. In many 

 Velellidae the stigmata lie so near the crest-basis that they are difficult to find. 



Trachese. — The number of tracheae arising from the lower face of the pneumatocyst 

 in the Velellidae is also much smaller than in the Porpitidse, but they are longer than in 

 the latter and usually branched. In many species eight tracheae arise from the periphery 

 of the eight radial chambers which surround the central chamber ; these are usually 

 branched (PL XLIII. fig. 6 ; PL XLIV. fig. 9). A small and variable number of other 

 tracheae arise here and there, irregularly scattered, from other parts of the pneumatocyst. 



The vertical crest of the pneumatocyst, or the sail-skeleton, wanting in Rataria, 

 exhibits different degrees of development in the various species of Velella and Armenista. 



i Kolliker, 4, Taf. xi. fig. 11. 



