220 
bulletin of the bureau of fisheries. 
at a point about midway on the back to a triangular depression, representing a tendon 
mark, and is thence continued forward on either side as a groove, which ends between 
the antennae and the mandibles. In a soft lobster a penknife can be readily inserted 
into this fold on the midline. Inwardly the pocket is continued into three divergent 
endotergites, which give attachment to parts of the posterior gastric muscles, but are 
absorbed previous to molting. Immediately below the forward end of the groove is 
seen the “grater,” a peculiar roughened area of the shell at the outlet of the branchial 
cavity; just before reaching this place the groove rises slightly, as if to avoid a promi- 
nent swelling, which marks the position of the ball of the outer hinge of the mandible, 
to be seen upon opening the branchial cavity. A branchio-cardiac line passes backward 
from each tendon mark toward the hinder border of the carapace, and with the cervical 
groove divides it into cardiac, gastric, and branchial regions. These lines are obscure 
in young animals, but become prominent grooves later, and deep furrows in lobsters of 
mammoth size. The gastric mill underlies the shell immediately in front of the cervical 
groove; a puncture behind this fold draws blood from the pericardium or the heart, 
while one below the branchio-cardiac line pierces the gill cover to the branchial chamber. 
The meaning of other tendon marks and muscle impressions on the carapace is given 
in a later section. Of the last io thoracic legs in the decapod, the first pair bear the big 
claws in the lobster and are its largest and most characteristic appendages. Its smaller 
and slenderer legs are chiefly ambulatory and sensory. The tail carries at either side 
on its under surface a bank of elastic oar-like feet of simple type, the swimmerets or 
pleopods for forward swimming, while the greatly enlarged and displaced sixth pair, 
or uropods, make with the telson the tail-fan already referred to. 
INTERNAL SKELETON AND HEAD. 
If we examine a well-prepared skeleton of a lobster we see that besides the outer 
hard crust there is a delicate internal skeleton, consisting not only of hard strap-shaped 
tendons at the joints of the limbs, but of a complicated linkwork of very thin plates or 
apodemes (pi. xxxm and xli). These unite to form partitions between successive sterna 
and their appendages in the cephalo-thorax, and form an internal or endophragmal 
skeleton. This intricate structure is produced by infoldings of the epidermal layer of the 
skin in the sternal and epimeral parts of the cephalo-thorax. The apodemes of which it 
is composed, are formed like the rest of the exoskeleton from matter secreted by the 
epidermis. Each plate or rod is thus double in origin, being formed in a flattened 
pocket like the tendons of the legs ( tp , fig. i, pi. xliii). 
According to Huxley “ four apodemes are originally developed as ventral folds of 
the skin between any two successive somites of the body, the anterior wall of each 
pertaining to the somite in front, and the posterior wall to the somite behind. These 
four apodemes thus form a single transverse series, the two nearer the middle line being 
called the endosternites, and the two farther removed the endopleurites. The linkwork 
“Huxley, T. H. The Crayfish, p. 158. New York, 1880. 
