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



logical function, since these Siphonophorse, which have no nectocalyces, descend into the 

 depths of the ocean by expelling the gas, and ascend again by secreting gas and filling the 

 float. We call this important new portion of the pneumatosac, which lines the inside 

 of the air-flask (excepting its apical portion), tapetum endocystale ("Secundaeres 

 exoderm," Chun, 48, pp. 514, 530). 



Hypocystic Villi.— The second peculiarity which distinguishes the air-sac of the 

 Rhizophysidse, Salacidse, and Epibulidas, is the production of peculiar hypocystic villi 

 (PL XXII. figs. 6-8, po ; PL XXIV. figs. 1-6, pv ; PL XXV. figs. 1-3, pv). These 

 remarkable apophyses of the air-funnel were first described in the Mediterranean 

 Rhizopihysa filiformis by Gegenbaur (7, p. 44, Taf. xviii. fig. 6, e) and by Huxley (9, 

 p. 6, pi. viii. figs. 14, 15); they occur not only in all Rhizophysidae, but also in 

 Salacia and Epibulia. From the hypocystic air-funnel, beyond the pylorus, arise eight 

 radial bunches of clustered villi, which fill up the basal portion of the pericystic cavity, 

 often more than half of it. The single villi, or the finger-shaped branches of the clustered 

 bunches, are composed of a single or a few gigantic exoderm-cells (1 to 2 mm. in 

 diameter) and of a ciliated epithelium of small entoderm-cells. Their function is probably 

 mechanical, as an elastic cushion to protect the delicate pneumadenia and prevents 

 its sudden compression. (Compare below the description of the float in the Rhizo- 

 physidse, and also Chun, 47, p. 404 ; 48, p. 529.) 



Siphons. — The feeding polypites or siphons in all CystonectaB are relatively large, often 

 of an extraordinary size. The four different segments of the siphon, which we could 

 distinguish in most Physonectae and Calyconectae (pedunculus, basigaster, stomachus, 

 proboscis), are also recognisable in many Cystonectae, as in Cystalia (PL XXII. fig. 5) and 

 Salacia (PL XXV. fig. 5). They are not distinguished, or at least not sharply separated, 

 in most Rhizophysidae, where usually each siphon is a simple cylindrical or fusiform 

 tube, distally contractile and protractile, with strong muscular wall (PL XXIII. figs. 1, 2, 

 5; PL XXIV. figs. 1-3, s). Probably in all Cystonectae the stomach, or the digestive 

 middle part of the siphon, bears inside numerous hepatic villi, sometimes arranged in 

 longitudinal series; but rarely there occur continuous hepatic striae, as in IAnophysa. The 

 glandular villi are often coloured brown or black by pigment-granules, especially in the 

 Physalidae (PL XXVI. fig. 6, sv). The stomach in these and in other Cystonectae is rather 

 distinctly separated from the proboscis, or the distal part of the siphon, without villi, with 

 thickened muscular wall. Its distal mouth-opening may be expanded in the form of a 

 large suctorial disc, usually of circular, more rarely of polygonal or quadrangular form. 

 The basigaster or the basal cavity (PL XXVI. fig. 6, sb) is usually not separated from the 

 stomach by a pyloric valve, and its exoderm is not strongly thickened ; it passes over into 

 the peduncle without a sharp boundary, and often this latter part is scarcely distinguishable. 

 But sometimes the peduncle of the siphon is rather long and thin (bp). The monogastric 

 Cystalidaa (PL XXII. figs. 1-5) possess only a single large siphon. All other Cystonectae 



