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



At first it is a low thin crest in the base of the soft muscular sail, and afterwards arises 

 as a hioh vertical lamella, usually of more or less triangular form ; the top of the triangle 

 is the highest point of the body, and placed in the upper or apical pole of the vertical 

 main axis. The broad base of the triangle, placed in one diagonal Hue of the parallelo- 

 gram-shaped disc,' is usually about one and a half times as long as each of the two equal 

 lateral sides. The substance of the crest is a thin chitinous plate, secreted from that part 

 of the pneumatosaccus which arises from the diagonal of the disc as a vertical fold. It is 

 perfectly homogeneous, solid, and structureless, and contains no canals nor air-chambers. 

 A number of lines or thin ridges, parallel to the two ascending edges of the triangular 

 crest, and visible on both flat sides of it, indicate its successive growth. 



Central Siphon. — The large central polypite of the Velellidae differs from that of the 

 Porpitidse and Discalidse in its bilateral compression ; the transverse section of its basal 

 part is circular in the two latter families, elliptical in the former ; the major axis of the 

 ellipse corresponds to that of the umbrella. Its general structure and shape are other- 

 wise the same as in the other Disconectse. The central siphon is comparatively large in 

 the small Rataria, where the number and size of the peripheral siphons is small, whereas 

 in the larger species of Velella and Armenista, where the peripheral siphons are very 

 numerous, the central polypite is less preponderant. The thick wall of the central 

 siphon is very contractile, and composed of two strong muscle-plates, an outer longi- 

 tudinal and an inner circular, both separated by a strong elastic fulcrum. The wall often 

 exhibits prominent radial or longitudinal folds, eight in the smaller, sixteen or more in 

 the larger forms. Correspondingly, the terminal mouth is often four-lobed or eight-lobed 

 (PI. XLIII. figs. 4, 8, so ; PI. XLIV. figs. 2-5, so). 



Centradenia. — The large central gland, or the so-called " central organ " (formerly 

 " liver"), exhibits the peculiar composition described above (p. 31). Bedot has given 

 an accurate histological description of its structure (59, 60). In the Velellidse it is more 

 developed than in the Discalidse, but much less than in the Porpitidse. It does not 

 usually occupy the greater part of the superior face of the subumbrella as in the latter, 

 but only its central part, between the centre of the float above and the base of the large 

 central siphon below. The outline and the horizontal section of the centradenia are not 

 circular, as in the Porpitidse and Discalidse, but elliptical or lanceolate, the major axis 

 of the ellipse corresponding to that of the umbrella. Its superior or apical face is more 

 or less conical, and fills up the concave inferior face of the pneumatocyst. Its inferior or 

 basal face is even, separated by the gastrobasal plate from the base of the central siphon. 

 The difference between the hepatic vessels in the superior half of the central gland, and 

 the renal vessels in its inferior half, seems to be usually not so striking in the Velellidse 

 as in the Porpitidse. The canal-plexus, as well as the compact parenchyma of exoderm 

 cells, which fills up the interstices of the canal-network, and probably secretes the gas, 

 is in the former far less developed than in the latter. This weaker development of the 



