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



unilateral development along the dorsal side of the trunk, the peculiar form and the 

 strange character of this family. Nevertheless the young larvae (Cystonula, PI. 

 XXVI. fig. 1) are not essentially different from those of other Cystonectse (e.g., Cystalia, 

 PI. XXII. figs. 1-5). The float is here a relatively small ovate or ellipsoidal vesicle in 

 the apical part of the primary medusome, connected by the inflated pedicle (a) 

 with the basal protosiphon (sa). The stigma on its apical pole (po) is the permanent 

 opening of invagination, from which the simple pneumatosac (or the inner float- wall) 

 depends freely into the pneumatocodon (or the outer float-wall). Chun tells us that the 

 float of these young larvae has a complete radial structure ("ein vollig radialer gebauter 

 ovaler Luftsack," 83, p. 559); but I have never found in the Physalidae (neither the 

 youngest larvae, nor older forms) any trace of radial structure; they possess neither the 

 eight radial bunches of hypocystic villi which are found in the Epibulida? (PI. XXII. figs. 

 6-8), the Salacidae (PI. XXV. figs. 2, 3), and the Rhizophysidae (PI. XXIV. figs. 1-5), nor 

 the corona of radial septa and pouches which is obvious in most Physonectae. The 

 pneumatosac of all Physalidae lies freely in the voluminous cavity of the large pneumato- 

 codon, and is connected with the latter only at the apical pole, by the sphincter of the 

 stigma. The pericystic cavity, therefore, is as simple as in the Cystalidae, Athoridae, and 

 Apolemidse. The outer wall of the float is very thick and muscular, with an outer layer 

 of strong, parallel, longitudinal fibres and an inner layer of circular ring-fibres. By 

 compressing the float voluntarily, the animal can extrude the included air through the 

 apical stigma, and sink down. After a short time has elapsed it can rise again, secreting 

 a great mass of gas by the pneumadenia, and filling the float. I often observed this process 

 repeated, in December 1866, off the Canary Islands. The lamellar pneumadenia, or 

 the glandular "air-plate" ("Luftplatte," Chun, 83, p. 569), which corresponds to the 

 endocystic tapetum of the other Cystonectae (p. 309), is a thin distal layer of exodermal 

 glandular cylinder-cells, placed originally in the basal portion of the pneumatosac. It 

 grows afterwards more towards the ventral side and extends forwards. It has a diameter 

 of 4 mm. in a float 20 mm. in diameter. But in the adult Caravella maxima it reaches 

 100 to 150 mm. in diameter, and occupies nearly the ventral half of the pneumatophore. 

 Asymmetry of the Corm. — The fundamental form of the bilateral corm is in all mature 

 Physalidae more or less asymmetrical, either the left or the right side being more 

 developed and organised differently from the opposite side. This asymmetry is less 

 expressed in the crestless Arcihxmidas (Alophota, Arethusa); it is much more prominent in 

 the crest-bearing Caravellidse (Physalia, Caravella). But the young monogastric larva 

 (Cystonula, PI. XXVI. fig. 1) is monaxonial, and only the tentacle arising from the 

 ventral median line marks the bilateral symmetry. As soon as the budding of the 

 secondary cormidia on the ventral side of the primary medusome commences, the 

 subvertical axis of the latter becomes more and more inclined, and finally subhorizontal. 

 The first trace of the asymmetrical development is then marked by the situation of the 

 v 



