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DEEP-SEA FAUNA. 



247 



sublittoral or mtermediate depths, while the most highly speciahzed forms 



•e characteristic of the very shallow waters. Such is the case with the 



are luoi 



Pn<ijurida3.* 



Doubtless in certain groups of lowly organized animals many species cast 

 in the antique mould survive in the abyssal depths of the ocean. But m 

 highly specialized groups, like the Stalk-eyed Crustacea^ 

 with visual and respiratory organs of a very perfect grade, — the peculiar 

 conditions that surround the dwellers of the deep work great structural 



beings endowed 



changes. 



Correlated with the retrogression of the visual organs, marked 



changes take place affecting the antenna) and anterior parts of the body 

 generally. The purity of the water in these still regions often leads to a 

 more or less complete disappearance of the epipods or *^' gill-scrapers. 



So it comes about that the Crustacea livhig at a great depth are apt to 



be rather specialized types, 



further removed from the primitive ancestral 



stock than are the allied species of the shore. Taking the animal kingdom 

 as a whole, it is probable that the archaic forms now extant in the shallow 

 waters of the land or coast, or in the moderate depths below the strictly 

 littoral zone, far outnumber those surviving in the extreme depths of the 



sea. 



m 



gula are all peculiar to shoal water. So are the Unionida? of the rivers and 

 ponds. Nautilus and Fleurotoniaria come from very moderate depths. The 

 Brachiopods, distributed from the shore-line to 2945 fathoms, attain their 

 maximum development in from 50 to 250 fathoms. The wonderful Crinoid 

 fields, — those lily beds of the Caribbean Sea^ — lie at a depth of but 50 to 

 200 fathoms beneath the surface. 



Only in a very broad and genei-al sense may the deep-sea Crustacea, 

 taken as a whole, be called antique types, inasmuch as they are to a very 

 great degree members of the Anomuran or Macruran series, — low in the 

 scale of classification, and in so far more primitive forms. Only four species 

 (representing two genera) of Brachynra were discovered by the " Albatrof^s 

 below 500 fathoms,! and these low in the Brachyuran scale. As bearing on 

 the suggestion of the boreal origin of the deep-sea Crustacea, it may be 

 observed that the Brachyura, that great group which scarcely tinges the 



* Milne Edwards and Bouvicr, in Ann. Sci. Nat, Zool,, 7 - Sc^r., XIII. 195, 1S92; Mem. Mus. Comp. 

 Zool, Vol. XIV., No. 3, p. 9, 1893. , . ■ ,■ ^r <!,„ 



t In ILc vast amount of nuilcrlal obtained by tlie " Challenger " during the circmnnavigation o tlie 

 globe only four species (belonging to tlu-ee genera) of Bracbynra came from below 500 fathoms, iwo ol Uicse 

 are the same as two of the four species secured by the " Albatross " below tho 500 fathom hue. 



