REPORT ON THE SIPHONOPHOR^E. 341 



tudinal axis, originally subvertical, becomes in this way inclined more and more, and 

 finally lies subhorizontally. 



The further development of the Physalidse is determined mainly by the multiplication 

 of the cormidia on the ventral or inferior side of the vesicular trunk, and by the progres- 

 sive extension of the pneumatosaccus along its dorsal or superior side. In Alophota and 

 Phijsalia, where the single large main tentacle is much longer than all the others, usually 

 the two groups of cormidia (the larger ventral and the smaller basal) remain separated, 

 and their further development is different. The smaller basal group, at the posterior or 

 distal end of the trunk, produces merely a series of small siphons and palpons, placed 

 before the protosiphon, and is provided with a single tentacle only ; it always remains sterile 

 and never produces gonophores. The larger ventral group produces early a very large 

 main tentacle, with a gigantic main palpon, much longer and stronger than all the 

 others. The number of cormidia in this ventral group is much larger, and the siphons 

 as well as the palpons and the accessory tentacles become very numerous in the larger 

 species. Some of them afterwards produce gonodendra. In Arethusa and Caravella, 

 on the other hand, the number of main tentacles increases, and usually the two groups 

 of cormidia (smaller basal and larger ventral) are early united into a single large mass of 

 crowded appendages. The succession and composition of the cormidia seem to follow, 

 however, somewhat different laws in the various species of Physalidse. 



The further development of the pneumatophore in the larvae of Physalidse has 

 recently been described by Chun (83, p. 559). The inferior or basal third of the 

 invaginated pneumatosaccus becomes separated from the superior larger portion by an 

 annular constriction. The cylinder-epithelium of the former is the pneumadenia, which 

 afterwards expands in the form of a gas-secreting "basal plate" ("Luftplatte"). Physalia 

 and Caravella afterwards develop the dorsal crest which is wanting in the float of Alophota 

 and Arethusa. 



Truncus. — The marked peculiarity in which the Physalidse differ from all other 

 Siphonophorae, is the strange development of the hypertrophic pneumatophore along the 

 dorsal side of the common trunk. The nectosome, therefore, occupies in this family the 

 entire dorsal half of the corm, whereas the siphosome takes the ventral half; the main 

 axis of the trunk becomes subhorizontal, whilst it is vertical in the other Siphonophorae ; 

 the nectosome occupies in these latter the apical or proximal, the siphosome the basal or 

 distal, part of the corm. The naked dorsal face of the trunk, which bears no appendages 

 but includes the float, is in all Physalidse much larger than the ventral face which bears 

 the cormidia of the siphosome. The cavity of the trunk is wide, and when the gas is 

 expelled through the stigma of the contracted float, the trunk appears as a voluminous 

 sac filled with nutritive fluid. The trunk in the Physalidse is never coiled up spirally as 

 in the allied Epibulidse. 



Pneumatophore. — The gigantic float of the Physalidse determines by its excessive 



