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



Order III. PHYSONECM, Haeckel, 1888. 

 (Pis. VIII.-XXI.) 



Physophuridx, Eschscholtz, 1829, 1, p. 139. 

 Physophorx, Goldfuss, Auctt. 



Definition. — Siphonophorae with a pneumatophore and several nectophores (or instead 

 of these bracts), without aurophore. Nectosome composed of a simple, apical, mono- 

 thalamous pneumatocyst, and beyond it a biserial or multiserial group of nectophores, 

 sometimes instead of these a corona of bracts. Siphosome always with numerous 

 palpons, usually covered with numerous bracts. The trunk of the siphosome is either a 

 single siphon (Monogastricae) or a tubular or vesicular stem, bearing numerous cormidia 

 (Polygastrieas). Each cormidium with a single siphon and a single tentacle, a group of 

 palpons and gonophores. The cormidia are sometimes ordinate, at other times irregular. 



The order Physonectae, hitherto usually called Physophoridae, comprises those very 

 numerous and variously organised Siphonophorae, which possess a nectosome composed of 

 an apical pneumatophore and a group of nectophores. They agree in this combination 

 with the following order Auronectae, but differ from these as well in the absence of the 

 aurophore as in the simple structure of the tubular trunk. Sometimes the nectophores 

 are replaced by bracts ; but these are also originally nectophores, with a reduced or lost 

 nectosac. A further character common to all Physonectae is the general presence of 

 palpons or cystons ; these are lacking in most other Siphonophorae, with the exception of 

 the Cystonectae. The corm of the Physonectae is rarely simple, and represents a single 

 cormidium (in the monogastric Circalidae and Athoridoe) ; usually it is composed of 

 numerous cormidia, which are sometimes ordinate (with free internodia), at other times 

 irregular (with scattered organs). The primary larva is always bilateral, and develops a 

 pneumatophore very early (Pkysomila). 



History. — The oldest descriptions and figures of Physonectae are those of Forskal 

 (1775), and relate to three Mediterranean forms, which he united in the genus Physo- 

 phora — Physopthora hydrostatica, Physophora rosacea, and Physopliora Jiliformis (11). 

 The first has been retained as the permanent type of the genus Physophora ; the second 

 was afterwards called by Eschscholtz (1) Athorybia, and the third Epibulia; this latter, 

 however, is a Rhizophysa, and belongs to the Cystonectae. 



In the beginning of the present century the first observations on Physonectae were 

 made by Peron and Lesueur. In 1807 they published figures of Physophora myzonema 

 and Stephanomia amphitrites (14, pi. xxix.). An excellent plate by Lesueur, with a 

 splendid figure of Stephanomia uviformis ( = Apolemopsis uviformis), was unfortunately 

 never published. Afterwards a number of detached portions of Physonectae, and fragments 



