REPORT ON THE SIPHONOPHOR^E. 53 



a stigma above, and a trachea (or a bunch of trachese) below. This evident octo- 

 radial structure of the central part is often lost in the peripheral part of the pneuma- 

 tocyst. 



The concentric ring-chambers which compose the latter are simple circular rings, 

 without radial septa. Their number is very variable, in the smaller species ten to 

 twelve, or even less, in the larger species forty to sixty, and in the largest more than a 

 hundred. They are usually of nearly equal breadth ; often, however, the chambers of 

 the middle part are wider than the more proximal (near the centre) and the more distal 

 chambers (near the margin). The first or innermost of the concentric ring-chambers is 

 different from all others ; it embraces the oetoradial central disc not only from the outer 

 (abaxial) but also from the lower (basal) side (PL XLVI. figs. 3, 6). 



Pneumothyrse. — It is a general opinion that the concentric circular ring-chambers 

 of the Porpitidse are perfectly separated one from another by solid annular septa. But 

 this is not the case. I found in all members of this family openings of communication 

 between them, which I shortly call pneuniothyrse. The thin chitinous annular septa, 

 which separate the ring-chambers, are concave on the axial side, convex on the abaxial 

 side, they are usually thicker in the upper, thinner in the lower, part. Each annular 

 septum is pierced in its basal part by at least eight pneumothyrse, ovate or roundish 

 openings, which are about twice as broad as the stigmata of the upper surface. The 

 pneumothyrse or ring-gates (PI. XLVI. figs. 3, 4, pg) lie originally in the same radii as the 

 stigmata (pe). There are, therefore, eight interradial rows of pneumothyrse. But in the 

 larger species their number is increased, accessory pneumothyrse being interpolated 

 between the primary ones in the outer chambers. 



The superior, proximal or apical side of the pneumatocyst is usually flat in the 

 central part, whilst the peripheral part is highly convex or campanulate in the 

 Porpalidse, slightly convex or even in the Porpitellidse. Its surface is sometimes smooth, 

 at other times rough and marked by radial stripes or ribs, aud by concentric circles. 

 It is sometimes, mainly in the central part, spiny, papillate, or armed with conical, 

 irregularly scattered tubercles. The chitinous substance in the central part is often 

 much thickened by apposition of secondary layers, and these may close the stigmata of 

 that part. Those of the peripheral part remain always open. 



The stigmata, or the pneumatic foramina on the upper face of the pneumatocyst (pe), 

 are much more numerous in the Porpitidse than in the Discalidse and Velellidse. Constantly 

 there is a central stigma in the apex of the central chamber, and around this a regular 

 corona of eight equidistant stigmata in the eight radial chambers which surround the 

 former. The other stigmata are usually not regularly disposed, but scattered in great 

 numbers over the upper surface. When the latter is provided with radial ribs or 

 prominent ridges, the stigmata are placed in the height of the ridges, not in the valleys 

 between them. The stigmata are sometimes simple openings in the upper wall of the 



