572 A NATURALIST ON THE "CHALLENGER." 



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 black coloured. It is closely 

 allied to the long-legged insects (Gerrys) 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 Ralobates 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 Mollusca 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-inhabit- 

 ing 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-Salpse 

 and Pyrosoma. In some of these compound organisms, such as 

 the Siphonophora, there is a complex combination of variously 

 modified zooids, with a complex division of labour amongst the 

 members composing the colony, just as amongst the closely allied 

 Stylastericlce. The Siphonophora like the Stylastericlce are Hy- 

 drozoa, 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 Pelagonemertes 

 Rollestoni, after my friend Prof. Eolleston of Oxford. 



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

 perfectly transparent, with the exception of the digestive tract, 



* See page 27. 



