244 
HISTORY OE BRITISH CRUSTACEA. 
Pam. SPHJEROMIDM 
Thorax of seven distinct segments, generally all capable 
of motion; seven pairs of legs almost equally developed. 
Abdomen very large ; the five first rings of the abdomen 
slightly developed, and generally united; theTast false legs 
terminating in one or two plates, the outer one only mov- 
able. Head large and transverse. The legs only adapted 
for walking. 
Most of the species can contract themselves into a ball 
when alarmed, and some of them have the power of making 
holes in the softer rocks. 
Gen. 124. SPHJEBOMA, Latr. 
Eyes not reaching the front margin of the first segment 
of the body. Body seven-jointed. Tail entire at the tip, 
the base on each side furnished with two equal leaf-like 
appendages ; outer plate of last false legs folded under the 
inner one. 
Dr. Johnston (Zook Journ. iii. p. 181) describes the S. ru- 
gicauda as swimming with considerable velocity, and often 
on its back. It contracts itself into a ball when alarmed ; 
and it can live a considerable time out of the water. 
