184 



THREE CRUISES OF THE " BLAKE. 



ill great numbers, forming regular windrows, and, when dry, are 

 blown by the winds to the highest parts of 

 the beach. 



Some of the structural features of the 

 Porpitidse indicate affinities with acalephian 

 corals like the Milleporidie, which date back 

 to the cretaceous, but their general liomo- 

 loo'ies ally them most closely with the tu- 

 bularian hydroids of to-day. Among the 

 finest siphonophores is a large Stephanomia, 

 with its bells arranged in many vertical 

 rows. 



One of the most interesting siphono- 

 phores is Pterophysa grandis. (Fig. 92.) 

 Of this species, which grows to a large 

 size, huge specimens measuring no less than 

 thirty feet, often came up on our dredge- 

 Avire in the Gulf of Mexico and the Carib- 

 bean. It is closely allied to those which 

 Studer, the naturalist 

 of the " Gazelle," re- 

 gards as strictly deep- 

 sea siphonophores. The 

 polypite of large speci- 

 mens often measured 

 two to three inches. 



Pterophysa and other 

 siphonophores have the 

 power of sinking and 

 then swimmino- back to 

 the surface, but neither 

 Velella nor Porpita ap- 

 pears capable of such 

 movements. A very 

 young Physalia, c o 1- 

 lected at the Tortugas, 

 was observed to swim at 



Fig. 92. — Pterophysa grandis. -jV- (Fewkes.) 



different levels in the jar in which it was kept. 



