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



ring-chambers. But usually they are dilated in the middle, more or less urn-shaped, 

 and sometimes prolonged into short prominent tubules (PL XL VIII. fig. 8, pe). 



The inferior, distal or basal side of the pneumatocyst, separated by the exodermal 

 epithelium of the pneumatosaccus from the neighbouring superior face of the centra- 

 denia, corresponds in its general form to the latter, and has therefore a more complicated 

 shape than its superior face. It is always more or less concave, hemispherical or 

 campanulate in the Porpalidae, slightly concave or cap-shaped in the Porpitelkdae. 

 Numerous radial ridges or folds are prominent from the inferior face of the float, and 

 often these arise in the form of vertical radial lamella?, separated by deep valleys. These 

 latter are filled up by corresponding high radial ridges of the upper face of the centra- 

 denia. The number of the radial ridges increases rapidly towards the periphery, 

 numerous secondary and tertiary centripetal lamellae (which do not reach the centre) 

 being interpolated between the centrifugal primary ones. The concave under surface of 

 the pneumatocyst, and the convex upper surface of the centradenia, catching one into 

 another, become very similar to a Fungia (PI. XLVIII. fig. 7). From the combs of 

 the prominent radial ridges arise the tracheae (fig. 6, pt). 



Tracheae. — The pneumatic tubules or aeriferous filaments, which we call shortly 

 trachea?, are much more numerous in the Porpitida? than in the Discalida? and Velellidae. 

 Each radial ridge bears in the larger species on an average more than one hundred tracheae, 

 and as the number of the ridges there amounts to some hundreds, the number of the 

 tracheae reaches many thousands. The innermost tracheae, nearest the main axis, arise 

 from the eight radial chambers which surround the central chamber. The other tracheae 

 arise sometimes isolated, in irregular radial series, from the crest of the ridges, at other 

 times in bunches, crowded in small groups of three to six or nine, rarely more. 



The aeriferous tubules are usually simple, very rarely branch, and never anastomose. 

 Their thin chitinous wall is cylindrical, of very different length. Their course is never 

 straight, always more or less curved, and often serpentine. The trachea? of all Porpitidae 

 are distinctly articulate, and seem to be composed of a series of short truncate conical 

 segments ; the distal or inferior end of each segment is wider (PI. XLVI. fig. 8). 

 Sometimes each segment seems to be separated from the other by a transverse septum ; 

 but the apparent septum is merely an annular constriction. The two ends of each ring- 

 segment (proximal and distal) remain always open. 



The course of the tracheal is difficult to make out. Descending from the lower face of 

 the pneumatocyst, they enter immediately into the centradenia, and run in sinuous curves 

 between the canal-network of this organ. A great part finishes inside the central gland, 

 whilst another part descends deeper and passes into the wall of the central siphon and 

 the peripheral siphons. They end here in the exoderm of the proximal half of the 

 siphons, and do not enter into their distal half. Usually the great majority of the 

 trachea? are much shorter than the vertical diameter of the centradenia ; they must there- 



