KONGL. SV. VET. AKADRMIENS IIAN!>L. BAND. 19. X:0 7. 87 



furthermost limits of solar light and of vegetation, in stillness and cold all but un- 

 varied, and where much of the available nutriment is contained in the sediment that 

 sparingly and slowly sinks down from tlie richer zones. Such are the abysses where 

 the Pourtalesiada^ have to lead their simple life, unapproached by many of the power- 

 ful agents at work above, and well may it be allowalile to surmise that the very poor- 

 ness of the conditions surrounding them, while leaving unawakened, and thus elimina- 

 ting a variety of tendencies forcibly active in the irduxbitants of more favoured regions, 

 has set free one deep-seated tendency, all Init dormant in them, the one towards a 

 structural differentiation ty])ical of a higher and different order, of the Annulat(! 

 Animals. 



The Pourtalesiadai are not alone among Echinoids to impart a peculiar aspect 

 to the abyssal fauna. Like them the Phormosomas are eminently deep-sea forms, the 

 ten species found by the Porcupine, the Challenger, the Blake and the Knight Errant 

 ranging from 219 metres to 5030 m., the average depth of the genus being 2625 metres. 

 In five out of the eleven Challenger stations where Pourtalesiadfe were dredged, they 

 were found associated with Phormosomas. Higher up these are represented by species 

 of Asthenosoma. 



As, among land animals and those of the sea that live near the shore, certain 

 types, properly belonging to and richly varied within the tropics, are found represented 

 in our temperate and even cold regions by one or the other of their forms, perhaps 

 somewhat modified, but still recognisable, thus also in the oceans, iti a bathymetrical 

 sense, certain types, properly littoral and highly developed in the favoured zones of 

 light, have outposts in the dark depths, sharing with true abyssal forms their reduced 

 conditions of existence. 



In the adult state most of marine Evertebrates remain at their native station, 

 wandering within its precincts. Their embryonic and larval age is their period 

 of dispersal. Of numerous littoral forms, of different classes, tribes and orders, 

 currents must occasionally carry away the free swimming larvse from the vicinity 

 of land far into the sea, and during the course of succeeding generations early 

 stages of many a species will in this way have reached the wide ocean. There they 

 will have sunk, their development accomplished, all through depths full of dangers 

 and more and more ungenial, and a few of them will have settled on the bottom of 

 the abyss, and fewer still Avill have come to thrive there. Among these some will have 

 long retained their original character, and but slowly been modified, while others will 

 have exhibited a latitude of variation unknown or rarely seen where they came from, 

 but upon the whole there will be reasons for assuming the less altered forms to be 

 newcomers, the more deviating to be old inhabitants of the deep. At present it is 

 too early to enter into these cjuestions, — Avhen the whole of the materials now on 

 competent hands shall have been worked out, and a general view of the facts obtained, 

 the time will have arrived for knowing, whether the abyssal fauna may be derivable, 

 in the way mentioned, as a whole, from the fauna of the littoral region, as from 

 ils original stock, both being recent and coeval, though widely separated, or, in part, 

 from littoral forms no more existing, but fossil not far off'. The results then arrived 



