REPOET OX THE HOLOTHUEIOIDEA. 
reniently this may he fitted out ; the difierences in temperature, pressure, saltness, &c., 
are too great and the change too abrupt. Thus I think that a slow continuous emiora- 
tion seawards is going on in the manner above sketched out, and there seems to be 
scarcely any doubt that the present deep-sea fauna will be changed in character in the 
course of time, chiefly on account of an emigration from the shore. 
It is rather peculiar that such an emigration is not always necessarily accompanied 
with any very obvious alterations of the organisation of the animals. Striking examples 
of this fact are afibrded, for instance, by those Cucumariw which are met Avith in the 
depths. Nevertheless it must be admitted that, as a rule, jiidgiug at least from the 
Holothurioidea, alterations of a more or less essential nature are produced during the 
migration. But, on the other hand, there are strong grounds for supposing that most 
forms which have once surmounted the difficulties of the migration, and reached a more 
considerable depth, are in several respects endowed with tlie faculty of preserving 
“certain” of the characteristics of their ancestors, i.e., the original shallow-water forms, 
longer than those descendants of the same ancestors winch, from being able to 
maintain the struggle for existence with greater advantage, have remained in the 
littoral region. Indeed, natural selection ought to have played a far more important 
role in the latter region than in the monotony of the abysses, and to have in general 
called forth swifter and more perceptible changes. It is within the shore region that the 
Synaptidae occur, which are of all Holothurids the most thoroughly modified and least 
Echinoderm-like in shape. Ea'cii the IMolpadidae, which are, in my opinion, somewhat 
older than the Synaptidae, are extremely altered forms, although, like certain Ciiciimario’,. 
they have emigrated at a later period, and seem now to thrive principally at con- 
siderable depths. 
Supposing that there is some truth in the hypothesis, which I shall attempt to prove 
below, that tlie common progenitors of all the Holothurioidea were not apodous Synapta- 
shaped animals, as has been hitherto asserted, but of the form of Cucumai'ice, and pro- 
vided with an open stone-canal, feet along the ambulacra, and a well-developed water- 
vascular system somewhat like that of the Ecliinids, it might appear. strange that even 
among the present shallow-water forms there are to be found species which have in 
some respects maintained themselves almost like the primitive form. On the other 
hand, it ought not to be forgotten that, notwithstanding this similarity, these very 
forms have undergone great and sweeping changes. They have lost the communication 
of the stone-canal with the exterior, the calcareous ring is highly altered, the tentacles, 
Avhich must originally have been simple and foot-like, have been transformed into very 
complicated dendroid organs, &c. And if we review the whole family of Dendrochirotse 
more closely, they are found to have varied in every possible direction so as to adapt 
themselves to the various modes of life that necessarily follow from the infinitely 
varying conditions of the littoral region. The Dendrochirotse have been split up, so to 
