380 GEOGRAPHICAL DISTRIBUTION. 



. Diphyes arctica has been treated at some length because the data allows us 

 to follow it over some twenty-five degrees of latitude, and because the question 

 of its probable vertical limits is germane to the ecology of Siphonophores in 

 general. But a considerable vertical range, irrespective of stage of development 

 or of season, has also been established for Diphyes appendiculata. This species 

 occupies a zone of considerable depth at all seasons in the Mediterranean, and 

 the same is true of it in summer in the Bay of Biscay. But the numerous records 

 of this species show that it is usually most abundant within seventy-five fathoms 

 of the surface (Chun, '87, Bigelow, :11b). 



Seasonal Fluctuations in vertical Distribution. 



Seasonal fluctuations in vertical distribution are known for several Siphono- 

 phores. The most important observations along this line are those made by 

 Chun ('87), and Woltereck's (:04) studies on the development of Velella. Chun 

 found that in the Mediterranean Physophora, Hippopodius, and Stephanomia 

 rubruni, which are common on the surface during autumn, winter and early 

 spring, seek deeper, and consequently cooler layers as summer approaches. 

 During the summer the larvae of all these were taken by him at from 100-900 

 meters (for a complete list of the seasonal occurrence of Siphonophores in the 

 Bay of Naples, see Lo Bianco, '99). In the Adriatic, the genus Praya is found on 

 the surface from January to the middle of April, and appears again by the end 

 of August; Stephanomia from October to February (Steuer, : 10, p. 571 ; Stiasny, 

 : 11). As I have already pointed out temperature is the factor governing the 

 vertical movements of these species, but this does not seem to be true of 

 Velella. The life-cycle of this genus, as traced by Woltereck, is as follows : — 

 it swarms on the surface in the Mediterranean at two periods, April- June, and 

 October-December (Lo Bianco, '99); and the sexual Medusae (Chrysomitra) , 

 set free on the surface, sink to considerable depths before becoming sexually 

 mature. The larvae (Conaria) then gradually rise by "die Bildung speci- 

 fische leichter Stoffe" to the surface, where they pass through the remaining 

 stages of development from the "Rataria" to the adult Velella. The fact 

 that our knowledge of Porpita has exactly the same gaps as did that of \"elella 

 until its larvae were collected off Villefranche, suggests that it probably carries 

 out the same vertical migration. And this may also be true of Physalia. 



Woltereck has suggested that the Chrysomitrae are negatively photo- 

 tropic, which could easily be tested, and no doubt soon will be; or they may 

 descend by simple passive sinking. 



