496 LIFE ON THE OCEAN SURFACE. 



was constantly caught during our voyage in the towing-net in 

 the open ocean. The Atlantic species differs from the Pacific 

 one. The insect is one of the Bug family, with a small round 

 wingless body and long legs, and is coloured black. It is 

 closely allied to the long-legged insects {Gerry s) which are so 

 commonly to be seen resting on the surface of ponds and 

 ditches in England, moving along by a series of jerks, and 

 casting curious-looking shadows on the bottoms of shallows 

 when the sun is overhead. The Halobates lives entirely at sea, 

 and carries its eggs about attached to its body. 



Most fish live about the coasts, and comparatively few are 

 met with far away from land, but there are regular pelagic fish. 

 \ There are pelagic MoUusca of all kinds, including perfectly 

 transparent Cuttle-fish, transparent pelagic Crustaceans, trans- 

 parent pelagic Annelids, and pelagic Planarian worms. 



There are even pelagic Sea Anemones {Nautactis and its 

 allies) which have their bases, by means of which shore- 

 inhabiting Sea Anemones cling to the rocks, so modified as to 

 form chambers containing air, and thus acting as floats. Many 

 pelagic animals form highly complex colonies, which float 

 about in the surface water, combined in one mass. Such 

 are Chain-Salpee and Pyrosoma. In some of these compound 

 organisms, such as the Siphonophorn, there is a complex com- 

 bination of variously modified zooids, with a division of labour 

 amongst the members composing the colony, just as amongst 

 the closely allied Sty laster idee. The SiphonopJiora like the 

 Stylasteridce are ffydrozoa, but the compound organisms they 

 form are soft, hyaline, and free-swimming, whilst the stocks 

 formed by the Stylasteridce are stony, hard, opaque, and 

 firmly rooted to the sea bottom. 



I have described a land Nemertine worm,* which exists 

 in Bermuda. Nemertines, however, though like Planarians 

 normally shore inhabiting animals, have adapted themselves 

 not only to terrestrial, but also to pelagic existence. One of 

 the most remarkable animals discovered by the " Challenger " 

 Expedition is a pelagic Nemertine which I have called 

 Pelagotiemertes Rollestoni, after my friend Prof. Rolleston of 

 Oxford. 



The body of the animal is leaf-shaped and gelatinous, and 

 perfectly transparent, with the exception of the digestive tract, 

 which is branched as in Planarians,! and is of a burnt-sienna 



* See p. 23. 



f Prof. Giard has lately described a gigantic Nemertine {AvenaTdia 

 Priei) a yard and a half in length, which has a similarly ramified intestine, 

 otherwise this arrangement does not occur amongst Nemertines^ Ann. 

 and Mag. Nat. Hist.; Sept. 1S78; 



