REPORT ON THE SIPHONOPHOR^E. 321 



nucleus" (9, p. 6, pi. viii. figs. 14, 15). This good description of Huxley was not 

 improved by later authors, who regarded the hypocystic villi as tubes or utriculi. 

 Fewkes (in 1882) describes them as "finger-like pouches, which are sometimes bifurcated 

 at their extremities and open at their distal ends, so that their cavities seem to 

 freely communicate with that of the float" (44, p. 269, pi. vi. fig. 2). The most 

 accurate description was afterwards given by Chun, who regards their physiological 

 function as mechanical ; they may serve as elastic cushions or bolsters, which protect 

 the delicate pneumadenia covered by them, and prevent its sudden compression, when 

 the stem is rapidly contracted (47, p. 404 ; 48, p. 529). My own observations on 

 the structure and development of these interesting villi, made in Lanzerote (1867), 

 and continued in Ceylon (1881), are in complete accordance with those of Chun. The 

 hypocystic villi are always arranged in eight radial bunches which arise from the outside of 

 the air-funnel ; each villus, or each finger dike branch of the dichotomously-branched villi, 

 consists of a single giant-cell, or a few (two to four, rarely more) giant-cells, which reach 

 a diameter of one to two mUlimetres, and belong, therefore, to the largest cells of animal 

 tissues ; the nucleus of these vesicular and vacuolated exoderm-cells is ovate or cup- 

 shaped, and has a diameter of 0"1 to 0'2 mm. The surface of the vdli is covered with a 

 vibratile epithelium, composed of small entoderm-cells with long cilia (PI. XXIV. fig. 6). 

 In the youngest Rkizophysidae there are only eight single club-shaped giant-cells, which 

 arise from the pylorus infundibuli ; they correspond to the base of the eight radial 

 apophyses of the air-funnel, which pass into the radial septa dividing the cavity of the 

 pneuraatophore into eight radial pouches in many Physonectae (e.g., Discolabida?, compare 

 above, p. 187). Afterwards arises a second corona of eight radial giant-cells from the 

 distal base of the hypocystic funnel, and a third corona between the former and the 

 latter (48, p. 530). By dichotomous ramification of these twenty-four giant-cells and 

 further development of lateral branches arises the large elastic cushion, composed of 

 numerous finger-like villi, which envelops in the adult Rhizophysidas the greater part of 

 the pericystic cavity and hangs down into the apical part of the stem-canal (PI. XXIII. 

 figs. 3, 4 ; PL XXIV. figs. 4, 5). 



Siphons. — The feeding polypites are in the Rhizophysidse usually of considerable size, 

 sometimes very large, 4 to 8 centimetres long, or more, in the expanded state. The four 

 segments of the siphon which are usually distinct in the majority of Siphonophorse are 

 rarely evident in this family ; in the majority they are not distinct or not recognisable at 

 all, so that the whole siphon is a simple cylindrical or spindle-shaped tube (PI. XXIII. 

 fig. 5; PI. XXIV. figs. 1-4). Sometimes, however, especially in the peculiar Linophysa, 

 the four segments are distinctly marked: — (1) A small pedicle to which the tentacle 

 is attached ; (2) a large ovate basigaster, the exoderm of which is full of cnidocysts ; 

 (3) a wide stomach with coloured hepatic glands ; and (4) a very contractile tubular 

 proboscis, with the tubular mouth-opening (40, p. 9, Taf. i. fig. 4). The thick wall of 



(zool. CHALL. EXP. — part lxxvii. — 1888.) Hhhh 41 



