ANATOMY OF THE LOBSTER 269 
and is so developed as to cover the other cephalo-thoracic 
segments, thus exemplifying, in an interesting way, Audou- 
in’s law of the development of one segment or part of a 
segment at the expense of adjoining parts or segments ; this 
law, so universal in the Arthropods, as well as throughout 
the animal kingdom, also applies to the appendages. 
The same parts are to be found in the crab, but in a modi- 
fied form, owing to the development or transfer of the weight 
of the organization headwards; in other words, the crab is 
more cephalized than the lobster; this is seen in the small 
abdomen folded under the large, broad cephalo-thorax, and 
in the greater concentration headwards of the nervous sys- 
tem of the crab. 
To study the internal structure of the lobster, the dorsal 
surface of the carapace and of each abdominal segment 
should be removed ; in so doing the hypodermis or soft inner 
layer of the integument is disclosed ; it is usually filled with 
red pigment cells. The dorsal vessel, or heart, lies under 
the hypodermis of the carapace, this being an irregular 
hexagonal mass surrounded by a thin membrane (pericar- 
dium) with six valvular openings for the ingress of the 
venous blood. The colorless, corpusculated blood is pumped 
by the heart backwards and forwards through three anterior 
arteries, one median and two lateral, the median artery passe 
ing towards the head over the large stomach, and the two 
lateral, or hepatic arteries, passing to the liver and stom- 
ach, From the posterior angle of the heart arise two 
arteries ; the upper, a large median artery (the superior ab- 
dominal), passes along the back to the end of the abdomen, 
sending off at intervals pairs of small arteries to the large 
masses of muscles filling the abdominal cavity ; the lower is 
the second or sternal artery, which connects with one extend- 
ing along the floor of the body near the thoracic ganglia 
of the nervous cord. The arteries become, at least in the 
_ liver, finely subdivided, forming a mass of capillaries. There 
are no veins such as are present in the Vertebrates, but a series 
of venous channels or sinuses, through which the blood re- 
turns to the heart. There is a large vein in the middle of the 
ventral side of the body. 
