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 pass- 

 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. 



