(3 
THE VOYAGE OF H.M.S. CHALLENGER. 
shown in my former work, but still it is clear, from the peculiar direction in which they 
have in general developed, that they cannot, since a very remote period, have had any 
connection with the other Holothurids, at least not with the Apoda, Ehopalodinidse, and 
Dendrochirotse. It is possible that the case is different with regard to the Aspidochirotse. 
In this fixmily the genus Stichopus, but above all the new genus Pcelopatides, brought 
home by the Challenger Expedition, presents in its whole organisation such a bewildering 
resemblance to some Elasipoda belonging to the family Psychropotidse, that it is almost 
impossible to find any other difference than the presence or absence of respiratory trees. 
Thus it does not seem improbable, though it cannot by any means be taken for granted, 
that the Aspidochirotse and the Elasipoda have sprung from a common branch, and have 
afterwards diverged from each other, the former losing the connection of the stone-canal 
with the exterior, &c., the latter losing the water-lungs, which must then be supposed to 
have existed in their common progenitor. 
Bathymetrical Distribution. 
Witli regard to the bathymetrical distribution of Apoda and Pedata, our present 
knowledge does not enable us to speak of any results of very general value. However, 
the Challenger Expedition has been successful even in these respects, several important 
discoveries having been made, proving that the present shallow-water fauna has far more 
outposts in the great depths of the ocean than at first supposed. Before the Challenger 
Expedition set out, only a very few forms belonging to the Apoda and Pedata were 
known from depths exceeding 100 fathoms, and scarcely one below 200 fathoms. 
The following list presents a view of the species met with in the deep sea at depths from 
500 fathoms and under (see p. 8). 
This list induces me to believe the following remarks to be true, or, at least, to 
have some prolmbility. 
1. Descendants of the recent shallow-water Holothurioidea have escaped to the 
greatest depths at which any living Holothurid has been obtained, viz., 2900 fathoms, 
but they are by no means so prevalent as the Elasipoda, nor do they form such a 
characteristic feature in the abyssal fauna. 
2. ]\Iost of the forms met wdth in the deep sea below 500 fathoms are distinct from 
the sliallow-water species, though they belong to the same genera. 
3. Several species have a vast bathymetrical distribution, some individuals of them 
still living near the shore, others having descended without any obvious change in 
their organisation into the considerable depth of 500 to 700 fathoms, or, exceptionally, 
even deeper. 
4. A wider distribution seawards of a species seems to take place preferably in the 
noiLliern and southern oceans, where the different belts proceeding from the vicinity of 
