BRITISH STALK-EYED CRUSTACEA. 139 
structure clearly indicates where this boundary ‘really occurs. 
The segments already referred to are termed somrrss, the lower 
wall of which is the srrrnum, to which in the abdominal region 
the swimmerets are attached. These last appendages each con- 
sists of a stalk or stem, which is composed of a very short basal 
joint, the coxopopiTE ; next to which is a long cylindrical second 
joint, the Basrpoprre; the whole stem being called the pRroro- 
popire. The swimmerets differ widely in different species, many 
of the Brachyura having them of a decided plumose structure—no 
doubt a necessary apparatus for the protection of ova, which, when 
discharged, are attached in a group by a viscid stalk to the coxo- 
podite, each mass of ova being shielded by the above-mentioned 
swimmerets. This formation of the ova in distinct groups is not 
general, but may be well observed in Hyas coarctatus. In others, 
however, they are attached severally and by separate ligatures ; 
whilst in others, again, they appear in the form of long strings 
radiating from a common centre, as in the case of Carcinus menas 
and Corystes cassivelaunus. 
The tail appendages, differing slightly as they do in different 
genera of Macrowra, may be broadly described as consisting of a 
fan-shaped arrangement of plates, the centre one being called the 
telson, which consists of two segments. The appendages on either 
side of this are more or less fringed with a hair-like substance, 
and the whole apparatus, together with the abdominal somites, 
are capable of very powerful muscular contraction, so that the 
rapid backward motion in the water, of many species, is thus 
attributable to this remarkable development. Scyllarus arctus and 
the members of the genus Galathea are particularly characterised 
by this power. So sure is their course through the water, and so 
complete their control over the muscular energy of this fan-shaped 
repeller, that Galathea strigosa has been observed to dart back- 
wards a distance of several feet, with most remarkable activity, 
straight into a small cleft in a rock, though the hole was barely 
large enough to admit its body. 
The Suborder under our consideration is termed Decapoda, 
from the fact to which we will now refer, namely, the existence of 
ten feet, or legs, otherwise termed AMBULATORY APPENDAGES. 
These ten legs also vary in the different tribes, and must therefore 
be considered accordingly; but taking the second ambulatory leg 
of Astacus fluviatilis, as given in Prof. Huxley’s work on ‘The 
