216 ANNULATA. 
accordance therewith, it is lined with a chitinous cuticle. 
In addition the cuticle in the stomach has a number of 
hard sclerites which form the gastric mil. This apparatus 
has a median tooth and two lateral teeth worked by power- 
ful muscles. Further, the aperture between the cardiac 
and pyloric portions is guarded by strainers, or small pro- 
cesses, covered with “hairs.” Digestion of the food is 
apparently confined to the region of the mesenteron. 
The sclerites of the lobster are moved by a complex series 
of muscles lying inside the body or limbs. There are two 
series of muscles—(1) the flexors which by con- 
traction bend the abdomen or the limb; (2) the 
extensors which straighten it. In the limbs, at least, these 
are attached to the arthrodial membranes by tendons, but in 
some cases to the edge of the sclerite. A cross-section of 
the abdomen shows the powerful flexors, the contractions of 
which bend the tail and drive the lobster backwards through 
the water, and above them the much thinner exdensors. 
The anterior flexors are attached in the thorax to the 
endophragmal skeleton, which consists of parts of the ecto- 
derm with cuticle; these have grown in from the ventral 
surface during development. Hence the endophragmal 
skeleton does not constitute an exdoskeleton. 
‘The lobster can swim gently forwards by the action of 
the swimmerets, it can creep in any direction by its legs and 
it can shoot rapidly backwards by contraction of the tail. 
The skeleton being an exoskeleton, it has already been 
noticed in the external features. We need only emphasise 
the tucking of the ectoderm into the stomach 
and into the ventral region of the thorax, the 
sclerites in each case forming the gastric mill and the endo- 
phragmal skeleton. 
The vascular systems of the lobster are in a peculiar 
condition. In the Annelida and Archicelomata we could 
distinguish two vascular systems. The larger and 
more spacious, contained z7¢him the mesoderm, 
was called the ccelom and was mainly nutritive and motor; 
the smaller consisted of fissures and small sinuses lying detween 
the three primary layers, was called the blood-vascular or 
hemocecelic system, and was usually respiratory and ex- 
cretory. 
Muscular. 
Skeletal. 
Vascular. 
