n6 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



there is a ganglion (really a pair) to each segment, how many ganglia should 

 there be in the cephalothorax of the lobster? Since the brain supplies the first 

 two pairs of appendages, how many appendages must be supplied by the sub- 

 esophageal ganglion, and of how many fused ganglia does it, therefore, consist? 



Trace the nerve cord back into the abdomen. Is there a ganglion for each 

 segment? Does the telson have a ganglion? Note the branches of the abdominal 

 nerve cord. 



Observe the branches of the sternal artery under the ventral nerve cord. It 

 passes between the two halves of the nerve cord, between the fourth and fifth 

 thoracic ganglia, and promptly divides into an anterior horizontal branch, the 

 ventral thoracic artery, and a posterior branch, the ventral abdominal artery. 

 These correspond to the ventral vessel of the earthworm. 



Put in the nerve cord and its ganglia accurately, and the branches of the 

 sternal artery on your drawing. The drawing is now complete. 



The nervous system of the lobster is very much like that of the earthworm, 

 except that as in the case of the other systems of the lobster it shows a partial 

 loss of the segmental arrangement through fusion. The nervous system of 

 both earthworm and lobster is based upon the same plan as that of Planaria, 

 consisting fundamentally of two ventral ganglionated cords arising from a dorsal 

 brain. This type of nervous system is common to all the invertebrates (except 

 those having radial symmetry) and is called the ladder type. The ventral cords, 

 originally separated rather widely, come together in the segmented animals to 

 produce an (apparently) single cord. 



3. General considerations on the lobster. The chief principle which we 

 wish to bring out through the study of the anatomy of the lobster has already 

 been emphasized throughout the laboratory instructions. It is that the lobster, 

 although a segmented animal like the annelids, exhibits a marked tendency to 

 an obliteration of the segmentation, through fusion and loss of segments and of 

 segmented structures. This tendency is still further in evidence in the verte- 

 brates where dissection alone reveals that the frog is segmented, and then in 

 only a few systems. What systems of the lobster are segmented? Which is 

 the most completely segmented? In what part of the body 'is the segmentation 

 most complete? Where it is most obscure? Compare with the frog and deter- 

 mine whether the systems and the part of the body which exhibit the greatest 

 degree of segmentation are the same in the two animals. Is it the same system 

 in the two which retains the most primitive segmentation? What is the signifi- 

 cance of this fact? Does the lobster have all the systems which are present 

 in the frog? What one does it possess which the earthworm lacked? Why is 

 this system the last to appear in the animal series? Are the systems which the 

 earthworm and the lobster have in common better developed and more specialized 

 in the latter? Which one differs the least from that of the earthworm? Do the 

 systems of the lobster compare favorably with those of the frog as to specializa- 



