170 
some species have the abdomen composed of four or five separate 
segments, other species with only two or three segments, and 
others again, like Idotea elongata , with the abdomen formed of 
a single piece.* The development of an operculum from the 
first pair of pleopoda would naturally follow from the flattening 
of the abdomen which would leave the pleopoda much exprcecl 
below. Thus Phreatoicus appears to have preserved the fully 
developed abdomen which must have been possessed by the 
ancestors of the Asellidw, while in the latter this has been 
specially modified in accordance with the general flattening of the 
body, which would render a long jointed abdomen unsuitable and 
a source of danger to the animal, especially by the exposure to 
which it would subject the pleopoda. 
There is one genus — Limnoria — sometimes classed under the 
Asellidw , which differs from them and resembles Phreatoicus in 
possessing an abdomen of six separate segments, the pleopoda also 
are unprotected. Limnoria however resembles the other Asellidce 
in the flat depressed body, and the segments of the abdomen though 
separate are short, so that Limnoria may very well be looked 
upon as an intermediate link between Phreatoicus and the Asellidce. 
The great difference between the two latter is caused by the fact 
that the body in the AseUidai is flat and depressed, while it is 
somewhat compressed in Phreatoicus , and that consequently the 
Asellidce are always represented as seen from above, while 
Phreatoicus is usually seen from the side. This difference in the 
form of the body though it gives quite a different appearance to 
the animal is probably not of very much importance from a 
systematic point of view, thus some of the species of Idotea differ 
very much in the shape of the body, and I think we must place 
Phreatoicus near to the Asellida but forming a new family, the 
Phreatoicida } , which bears to the Asellidce much the same relation 
that the Caprdlidce do to the Ct/amidce among the Amphipoda. 
Limnoria will perhaps best be placed as a special subdivision of 
tli e Asellidce, connecting them to some extent with the Phreatoicidce. 
I have not compared Phreatoicus with some other families of 
the Isopoda that it might well be compared with, such as, for 
intance, the uEgidte and the Sphaeromida J , but from what has been 
already said it will readily be seen that it possesses various 
characters in common with these as well as with those already 
considered inasmuch as it preserves to a large extent the typical 
characters of the Isopoda, and thus occupies a more or less central 
position, around which the other families may be grouped. 
The relation of Phreatoicus to its nearest allies may be graphi- 
cally represented by the following hypothetical genealogical tree: 
* See <k Revision of the New Zealand Moteid®,” Transactions N. Z. Inst.,, 
XXII., p. 199. 
