IO CRUSTACEA——-EUCARIDA—DECAPODA CHAP. 
habit (Galathea, Fig. 116), but often go down into great depths 
(Munidopsis, Fig. 114). 
Fam. 3. Porcellan- 
idae.—The abdomen is 
folded against the thorax, 
and the body has a crab- 
like form. © “These are 
always littoral in habit, 
never descending into the 
depths.  Pachycheles in 
the tropics,  Porcellana 
with numerous species in 
all seas, P. platycheles 
being a common British 
species. 
Tribe 2. Hippidea. 
. The Mole-crabs have 
\ ila tel the habit of burrowing in 
Fic. 116.—Dorsal view of Galathea strigosa, X 4. sand, and their limbs are 
Gee | original figure prepared for Professor peculiarly modified- into 
digging organs for this 
purpose (see Fig. 117). In other respects they are seen to be 
closely related to the Galatheidea by the form of the carapace, 
the condition of the abdomen, and the reduced last thoracic limbs. 
In Albunea, which is found in the Mediterranean, the first 
antennae,’ are greatly lengthened and apposed to one another, and 
by means of a system of interlocking hairs they form a tube 
down which the water is sucked for respiration. The object 
of this arrangement is to ensure a supply of clear water, filtered 
from particles of sand, when the crab is buried beneath the 
surface, on these occasions the tip of the antennal tube being 
protruded above the surface of the sand. An exactly similar 
tube is used by the true Crab Corystes cassivelaunus, which has 
similar burrowing habits, but here the tube is formed from the 
second antennae and not from the first, so that the tubes in the 
two cases afford beautiful instances of analogous or homoplastic 
structures between which there is no homology (see p. 189). 
1 Garstang, Quart. J. Micr. Sct. xl., 1897, p. 211. 
