ALLMAN. — USE OF THE TOWING NET. 59 



of them are provided with a delicate transparent shell, others are 

 quite naked. Some of the naked forms {Clio) have long been 

 associated in the accounts of Arctic voyage with the fauna of high 

 northern seas, where they occur in immense numbers, and are be- 

 lieved by the whale fishers to form the principal food of the whale. 

 They are easily taken in the towing net, are of considerable 

 consistence, and can be well preserved in spirit. 



The Heteropoda {Carinaria, Atlanta^ Firola, &c.), another free- 

 swimming form of the higher Mollusca, are also either naked or 

 provided with an external shell, which may be large enough to 

 enclose the entire animal, or be only sufficient for the protection 

 of the respiratory and reproductive organs. They swim by means 

 of a vertical fin, which projects from the ventral surface. They 

 are abundant in the warmer temperate, and equatorial seas. 

 Their habits resemble those of the Pteropoda, and they may be 

 captured and preserved in the same way. 



Some of the Nudibranchiate Gasteropodous Mollusca also possess 

 pelagic habits. It is rare, however, to find among them free- 

 swimming species, and they are mostly indebted for their pelagic life 

 to floating sea weed (Gulf weed, &c.) on the fronds of which they 

 habitually live, and by which they are carried about from place to 

 place in the open sea. Floating sea- weed, indeed, ought always 

 to be carefully examined. It frequently affords a rich storehouse 

 of rare animals, which are for the most part easily preserved in 

 spirit. 



Among the Mollusca are also many free-swimming larval forms. 

 These are all minute animals, generally furnished with a pair of 

 conspicuous wing-like swimming organs, and with a little nautilus- 

 like shell. Though proceeding from more or less sedentary 

 parents, their life in this stage is entirely that of the free-swim- 

 ming Pteropods, and they become easy captives of the towing net. 

 They must be removed by the dip tube, and preserved in spirit. 



Vertebrata. 



We can hardly expect to meet with vertebrate animals among 

 the contents of the towing net. Occasionally, however, small fishes 

 (Syngnathidag, &c.) frequent the most superficial zone, and will 

 be captured in the net. Small fishes of pelagic habit are not 

 unfrequently taken among floating sea weed. All these should 

 be preserved in spirit. The occurrence of floating fish-eggs should 

 be noted, and specimens reserved. 



Phosphorescence. 

 It is now well ascertained that the Phosphorescence of the sea 

 is mainly due to living animals which frequent by night the more 

 superficial zones ; and no opportunity of carefully observing this 

 phenomenon ought to be neglected. It is of importance to know 

 the various species to which the light-giving function must be 

 assigned, and to determine the conditions which may aid the 

 luminosity or interfere with it. The collector should always make 

 a note of the possession of this property by any animals in which 



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