REPORT ON THE SIPHONOPHORvE. 349 



Genus 746. Arethusa, 1 Haeckel, 1888. 



Definition. — Physalidse with a simple vesicular pneumatophore, without dorsal 

 polythalamous crest. Siphosome with several large main tentacles of about equal size. 



The genus Arethusa was established a century ago (in 1789) by Patrick Browne, for 

 that gigantic Physalid of the Tropical Atlantic, which is known to the sailors as the 

 "Portuguese Man-of-War," and which 0. F. Miiller and Gmelin had called Medusa 

 caravella (afterwards Salacia), the type of our genus Caravella (Genus 756). Since the 

 generic name Arethusa was afterwards given up and replaced by Lamarck's name Physalia, 

 we employ here the former for the designation of those Physalidffi which agree with 

 Caravella in the possession of numerous large main tentacles, but differ from it in the 

 absence of a polythalamous crest on the pneumatophore. Arethusa exhibits, therefore, 

 the same relation to Caravella as the crestless Alophota bears to the crested Physalia. 



Two different species of Arethusa were observed by me ; the first, Arethusa thalia 

 (from the Indian Ocean), is similar to Alophota giltschiana (PI. XXVI. fig. 3), and 

 exhibits a free interval between the single basal cormidium at the distal end of the 

 trunk and the central group of loose cormidia on its ventral side. The second species 

 (inhabiting the Atlantic, and figured in PI. XXVI. figs. 4-8) does not exhibit that interval, 

 but all the cormidia form together a single large group in the posterior half of the ventral 

 side of the trunk. 



Arethusa challenged, n. sp. (PI. XXVI. figs. 4-8). 



Habitat.— North Atlantic, Station 354, May 6, 1876; lat. 32° 41' N., long. 36° 6' W.; 

 surface. 



Canary Islands, Lanzerote ; December 1866 (Haeckel). 



Corm (fig. 4, lateral view of the mature corm from the right side, in the expanded 

 state; fig. 5, from the left side, in the contracted state). — The largest corms observed which 

 possessed gonodendra at the base of the siphons had a length of 40 mm. to 50 mm. 

 The colour of the corm was greenish-blue ; the common trunk and the basal ampullae 

 light greenish, their tops often reddish ; the siphons dark blue, with black villi, their 

 proboscis often reddish ; the tentacles partly blue, partly purple. The clustered 

 gonodendra (placed on the right side) had a reddish colour. 



Pneumatophore (figs. 4, 5, pf). — The float filled with air occupies the greatest part of 

 the trunk ; it exhibits as usual very different forms, according to the variable state of 

 contraction. The longitudinal axis is subhorizontal. The outer wall (pneumatocodon) 

 is separated from the inner wall (pneumatosac) by the cavity of the trunk, which is 

 rather wide and sacciform in the posterior half, and especially on the ventral side. The 



1 Arethusa = ' ApiSovox, a Nymph, daughter of Nereus. 



