1442 
THE VOYAGE OF H.M.S. CHALLENGER. 
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riir XoRTUKRN 
am* SorTiirRN* 
Sars says: “Concerning tlio geographical ilistribution of this species \_Lopliogaster tijpicus\ the 
occurrence of this form in the southern hemisphere, as shown by the Challenger collection, is remarkable, 
anil might induce the belief that it ranges from the Norwegian Sea along the whole western coast of Europe 
and Africa, or throughout the boreal, lusitanic, tropic, and antiboreal regions. It may, however, be considered 
as a highly remarkable fact, that this very striking form has never been recorded either from the coasts of 
England and France, or from the Mediterranean, although each of these tracts has been carefully investigated 
by numerous zoologists. AVe may therefore entertain the assumption that this form in reality does not occur 
throughout the intermediate tracts of the ocean, but is met with independently in both hemispheres in the 
corresponding region. Should this be the case, we may infer that the distribution of the species must at 
an earlier date have been continuous, but considerable changes afterwards occurring in the physical conditions 
led to a separation of the species into two independent stocks. In the sequel we shall meet with another 
still more striking e.xample of a similar kind, in treating of the Mysidan Boreorngsis scyphops, a form stated 
to occur in the Arctic and Subantarctic regions only, having never yet been found in any intervening tract. 
. . . The typical species [of Boreomysis^ is Borromysis arctica, first met with in the Arctic Sea, off Greenland, 
and also occurring along the coasts of Norway, where two other species, Boreomysis tridens and Boreomysis 
tnfgalops, have been also observed. Moreover, on the Norwegian North Atlantic Expedition two large species 
were taken, Boreomysis nohilis and Boreomysis scyphops, the latter of which is also represented in the 
Challenger collection [from the Southern Ocean]. Finally, there are two additional species from the 
Challenger Expedition [one from the North Pacific and one from the North Atlantic]. . . . The total number 
of species thus amounts to seven. All are true deep-sea forms, the animals descending to very considerable 
depths, and having never been met with in shallow water. . . . The geographical distribution of this form 
[Boreomysis scyphops] is very remarkable, ranging, as it does, within the limits of well-nigh the same region 
in both hemisplieres, and not occurring in the intermediate tracts of the ocean. As it cannot be reasonably 
assumed tliat tlic species has originated independently in both hemispheres, the j^hysical condition of the 
sea-bottom must at some time or other have been more uniform than at present, to have admitted of the 
species spreading over a much more extensive area, whereas at a later period essential changes in the 
climatological conditions must be assumed to have caused this form to withdraw successively from the equa- 
torial region towards the two poles, thus dividing the species into two widely separated stocks, inhabiting 
corresjmnding regions in both hemispheres. Another fact, too, viz. , the remarkable occurrence ... of the northern 
form, Lojihogaster typicus, in the Southern Ocean, without its being ever met with in the intermediate tracts, 
may also warrant the assumption of essential changes in the physical conditions of the sea-bed having taken 
place at some former period, thus causing the occurrence of certain species to appear discontinuous.”^ 
Sars further writes : “ The development of most Schizopoda exhibits a very striking resemblance to that 
of the Isopoda, the young passing within the marsupium of the female through one or more so-called pupa- 
stages before being liatched. In the Euphausiidie, however, a totally different mode of development has 
licen discovered, the young of these animals being hatched in a very immature condition, and not attaining, 
till after an exceedingly complicated free metamor[)hosis, the form characteristic of the adults.” - 
Sab.*) writes: “The genus [Leucon] at present comprises, besides the two new forms described below [one 
from Kerguelen and one from the North Pacific], nine species, seven of which are from the Northern Ocean, 
one from the Mediterranean, and one from the Atlantic. . . . The genus [Diastylis] is very abundantly 
represented, especially in the Northern Ocean, no less than twenty-seven distinct species having been hitherto 
roconlwl. The Challenger collection contains four species, one of which [from the North Atlantic] has been 
formerly dcscriljed, whereas the three others [two from the North Atlantic and one from Kerguelen] are now 
oddeil, increasing the numl)er to thirty-one.” ^ 
1>EunARr>, 8[ieaking of the development of spines in Isopods, writes : “ This modification of structure is 
not, however, confined to the dcef>-sca species ; it is found in quite as marked a degree in Arcticrus furcatus and 
Arrtuna sttui^ and other species of Ardurus- it will be noticed, however, that these species are inhabitants 
of the colder regions, and indeed it would appear that there is some connection between temperature and the 
' Zool. Cball. Exp., pt. xxzvii. pp. 16, 177, 178, 182. ® Zool. Chall. Exp., pt. x.vxvii. p. 8. 
* Zool. Chall. Kxi>., pt. Iv. pp. .33, 41, 
