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



Cormidia. — The entire body represents in the monogastric Cystalidae (PI. XXII. figs. 

 1-5) a single cormidium, composed of a large central siphon with a single tentacle, and 

 at its proximal base (beyond the pneumatophore) a corona of palpons, and a single 

 gonodendron. This solitary cormidium is similar to that of the Circalidas and 

 Athoridse (PI. XXL), but differs in the absence of nectophores and bracts. The four 

 other families are polygastric, and therefore composed of a variable number of cormidia. 

 These are sometimes ordinate, at other times loose. They exhibit a different shape 

 in the two subfamilies of Ehizophysidse, where the trunk is very long, tubular, and 

 articulate ; the cormidia are ordinate and Separated by equal free internodes in the 

 Cannophysidse (PI. XXIV.), where each cormidium is composed of a single siphon with 

 its tentacle, and a single gonodendron at its base ; whereas the cormidia are loose in 

 the Linophysidse (PI. XXIII. ), where the gonodendra are detached from the siphons, 

 usually alternating with them regularly. The family Salacida3 (PI. XXV.) is distinguished 

 by polygastric ordinate cormidia ; the long tubular stem bearing in regular free intervals 

 numerous equal cormidia, each of which is composed of several siphons (with tentacles) 

 and gonodendra. The shortened vesicular stem of the Physalidae (PI. XXVI.) bears 

 clusters of loose cormidia, without regular arrangement ; the cormidia of the Epibulidse, 

 however (PI. XXII. fig. 6), seem to be ordinate, similar to those of the Discolabidse. 



Pneumatophore. — The hydrostatic apparatus or the swimming-bladder is in all 

 Cystonectse large, sometimes gigantic and larger than all the other parts of the body 

 together ; it is the more important, as it is the only organ of floating, the nectophores 

 being entirely wanting. In Physalia it attains unusual dimensions (one or two decimetres 

 and even more), and occupies a far larger volume than in any of the other Siphonophorse. 

 The general form of the pneumatophore is usually more or less ellipsoidal, ovate or 

 pyriform, sometimes more irregular, oblongish round. Its longitudinal axis is usually 

 not vertical (as in the majority of Physonectse), but more or less obliquely inclined, 

 and in Physalia nearly horizontal. The cavity of the pneumatocyst opens in all 

 Cystonectse by an apical stigma, or a permanent pore, by which the animal expels 

 the included gas at will, when it will sink down. I have observed this emission of 

 gas repeatedly in the Rhizophysidse, Epibulidse, and Physalidse. 



Pneumatocodon. — The outer wall of the pneumatophore, which we call pneumatocodon 

 ("Luftschirm"), is in all Cystonectse very thick and muscular, and capable of strong 

 contraction. The strongest muscles are the longitudinal or meridional fibres, which lie 

 immediately under the exoderm and diverge from the open apical pole to the equator, 

 and then converge from the latter to the basal pole. They form around the apical stigma 

 a strong musculus dilatator (like that of the pupil of the eye). But further the stratum 

 of circular muscles of the entoderm, which is antagonistic, and separated from the 

 former by the thick fulcrum, is well developed. It forms around the apical stigma 

 a strong sphincter, or a circular musculus orbicularis, which closes its opening completely. 



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