MARINE CRUSTACEANS. 695 
usually broad, forming a cover to the mouth, whereas those of the Thalassinidea and Anomala 
are only exceptionally so. These characters, however, are in themselves hardly enough to 
negative the supposition that the Crabs have originated from one of the other non-macrurous 
groups. What does make it impossible to derive them from the Anomala is the presence 
in the primitive crab Homolodromia of podobranchs on some of the legs, while an origin 
from the Thalassinidea is equally untenable on account of the less primitive condition of the 
grooves of the carapace and the reduction of the endopodite and flagellum of the first 
maxilliped in the latter group. 
We are now left with the Thalassinidea and the three divisions of the Anomala. still 
unaccounted for. Among these there can be no question that the Thalassinidea are, as a 
whole, the most primitive. Their straight symmetrical abdomen, with well-developed biramous 
limbs on all the segments except the first, and the sixth pair broad and flat in all but 
Thalassina, the presence in most of appendices internae, and in some of good pleura, the 
simple legs of the fourth pair and sometimes also of the fifth, the rostrum usually well- 
developed, the moveable antennal scale seldom wholly lost, and the large gill-formulae of 
many genera, extending from the second maxilliped to the fourth leg, with mastigobranchs, 
podobranchs, arthrobranchs and sometimes pleurobranchs 

all these point to the same con- 
clusion. Nor can there be any doubt that the Paguridea and the Thalassinidea are closely 
akin and have branched off from the same not very remote ancestor. The only differences 
between the Axidae and the primitive Pagurids with symmetrical abdomen such as Pylocheles 
are the presence of a pleurobranch on the last leg, a suture across the telson in some species 
(which is never found in Thalassinidea), better developed eyes!, the loss of epipodites on the 
legs, a reduced rostrum, subchelate legs of the fourth pair, and the branches of the last pair 
of abdominal limbs narrow and not adapted for swimming. The first three of these features, 
being primitive, show that the Paguridea are not to be derived from any of the existing 
Thalassinidea and the remainder make it equally impossible to derive the latter group from 
the former. The two meet at a point below our present horizon. 
The Galatheidea join the stem from which the last two groups arise before its 
bifurcation, that is, they are less closely akin to either of these than the latter are to one 
another. The fact that they may have epipodites on some of the legs, and the more primitive 
shape of the rostrum and sixth pair of abdominal appendages make it impossible to place 
their ancestor within the present limits of the Paguridea. The suture on the telson and 
the pleurobranch on the last leg remove it also from the Thalassinidea. And the general 
shape of the body, depressed, with broad flat abdomen carrying long pleura and bent under 
the thorax, is so different from the compressed body, with straight abdomen, of the primitive 
Paguridea and Thalassinidea that there can be little doubt that the ancestral Galatheid left 
the non-macrurous stock, after the Crabs indeed, but before it gave rise to the Thalassinids 
or Hermit-crabs. As for the linea anomurica, this is found not only in the Galatheidea and 
Paguridea but also very distinctly in Callianassa novaebritanniae, and it is curious to notice 
that it is wanting in the primitive Pagurids such as Pylocheles, and most Thalassinids as in 
Azius, and that in the Galatheid Aeglea, which is also primitive in many respects, the hinder 
part is again wanting. Thus it would seem as though this line appeared only in the higher 
and more typical forms of each group. The Hippidea present a very difficult problem. On 
1 Such species as Awiopsis clypeatus, however, have the eyes well developed, though not so large as those of the 
Paguridea. 
89—2 
