dp:ep-sea fauna. 247 



sublittoral or intermediate depths, while tlie most highly specialized forms 

 are more characteristic of the very shallow waters. Such is the case with the 

 PaguridiB.* 



Doubtless in certain groups of lowly organized animals many species cast 

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

 highly specialized groups, like the Stalk-eyed Crustacea, — beings endowed 

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

 conditions that surround the dwellers of the deep work great structural 

 changes. Correlated with the retrogression of the visual organs, marked 

 changes take place affecting the antennce 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 living 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. Heterodonta, the Ganoid fishes, Limidus, PoUicipes, IVigonia, and L'm- 

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

 ponds. Nautilus and Plcurotomaria 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 general 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 Anonuu-an or Macruran series, — low in the 

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

 (representing two genera) of Brachyura were discovered by the "Albatross" 

 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 wdiich scarcely tinges the 



* Milne Edwards and Bouvier, in Ann. Sci. Nat., Zool., 7 '"= Sen, XIII. 195,1892; Mem. Mus. Coitip. 

 Zool., Vol. XIV., No. 3, p. 9, 1893. 



t In tli'e vast amount of material obtained by the " Challenger " during the circumnavigation of the 

 globe only four species (belonging to three genera) of Brachyura came from below 500 fathoms. Two of these 

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



