> 
Which follows it so that it may be covered by the posterior 
_and the surface of the ring. 
to hold good viz.: that the movable part is pulled into the 
_Ibacus, the first pair, and in fact all of the thoracic limbs end - 
gestion of a chela. In Crangon, (fig. 3) on the other hand 
OS me “hand? of & form of Cre 
-o a “eda eof ie Increase of the muscular po 
94 The American Naturalist. | [Februray, 
of the segment itself. In some of the Scyllaridw, there isa 
transverse groove on the terga of the somites merely indicated — ; 
in the lobster, into which the posterior face of the terga ahead _ 
of it fits when the abdomen is fully extended, and which pre- 
vents any further upward movement. It will be noticed 
further, in those forms that the sternum of each abdominal | 
segment is very narrow, and that they are connected by a _ 
broad uncalcified membranous area which allows plenty of 1 
room for flexion downward. ; 
The first abdominal segment is narrow, and in the vast | 
majority of cases it is considerably narrower than the one — 
ce 
edge of the carapace and the anterior edge of the second 
abdominal ring. In the Homaridea and Loricata there is an a 
interesting provision to prevent the strain of the powerful 
abdominal muscles from drawing the smaller segment too far 
into the thorax. On each of the lateral faces of the ring there 
is a broad tooth or calcified lobe, which overlaps the posterior — 
edge of the carapace allowing the latter to play between it 
When the limbs are examined, the same rule will be found 
fixed part. A modification of this is well illustrated in the 
evolution of the large chelæ. In some forms, take for example 
in a sharp pointed segment, there being not the slightest sug- 
the ternimal segment is pulled 
against the broad face of thé 
penultimate one thus making 4 
shift for a chela. In the Stom- 
~K ~ ` and forms with ita very effective 
6 figs ~ grasping organ. The continual 
| use of the terminal segment, the 
