376 BULLETIN OF THE BUREAU OF FISHERIES 



a large Bolinopsis is completely absorbed by a Beroe in four or five hours' time 

 (L. Agassiz, I860, p. 274). Copepocls, also, are often found in its digestive cavity. 

 Beroe, like all the other pelagic animals that inhabit the gulf throughout the 

 year and are widely distributed there vertically as well as horizontally, necessarily 

 experiences nearly the whole gamut of temperatures and salinities that prevail there 

 at one season or another; and although its habit of sinking in winter results (whether 

 voluntarily or not) in its avoiding the very coldest water, with 2 to 3° the lower limit 

 to its regular occurrence in the gulf, it has been found living actually among the ice 

 in the Arctic Ocean (Mortensen, 1912), apparently thriving, to judge from the large 

 size of the specimens in question. Nor does heat act as a barrier to its vertical migra- 

 tions within the extremes normal to the gulf — witness how often it comes to the 

 surface on calm days in summer and how abundantly it spawns at that level at the 

 season when the gulf as a whole is at its warmest. Beroe is equally catholic with 

 respect to salinity, except that it has not been found in the very freshest water of the 

 gulf at the time of the spring freshets — that is, in salinities lower than about 31 per 



mille. 



Other ctenopliores 



No other ctenopliores have actually been recorded of recent years within the 

 geographic confines of the Gulf of Maine as here limited. Another lobate species, 

 Lesueuria hyboptera, was described by A. Agassiz (1865) from Massachusetts Bay, 

 but has never been seen since. Mayer (1912, p. 20) has suggested that it was actually 

 Bolinopsis with the oral lobes torn off and the edges healed over to produce a rounded 

 contour, he having seen many in that condition in Halifax harbor after a storm. Its 

 status remains problematical. 



Mnemiopsis leidyi, a southern neritic form very abundant along the coasts of the 

 middle Atlantic States, is common as far north as the Woods Hole region during some 

 summers, but it has never been known to round Cape Cod. 



The Venus' girdle (Cestum veneris) was taken off the southeastern slope of 

 Georges Bank in 1872, among an assemblage of other tropical plankton (Smith and 

 Harger, 1874). 



SlPHONOPHORES 



Although the siphonophores are well represented in the warm oceanic waters 

 off the continental slope abreast of the Gulf of Maine, only one member of this 

 group of oceanic ccelenterates — Stephanomia cara — is anything but a rare stray 

 within the latter. It is probable that the low salinity of the gulf, as much as its 

 comparatively low temperature, makes it inhospitable to siphonophores, for, as I 

 have previously pointed out (Bigelow, 1911a, p. 3S1), they "are almost a negligible 

 factor in the plankton in waters with a salinity less than 35 per mille" and "are 

 entirely absent when the salinity is below about 30 per mille," a generalization that 

 applies as well to the North Sea region on the eastern side of the North Atlantic 

 as to North American coastal waters on the western. 



