THE CRAYFISH 147 



answer this if you understand how the somites of the abdomen are 

 articulated to one another. The appearance of the region between the 

 heart and gastric mill differs with the sex and sexual maturity of the 

 specimen. In a female, if the ovaries are well developed, they will be 

 seen as a paired mass in front of, and as a single median mass behind, 

 the heart. In a male the testes are less conspicuous but have the same 

 general Y-shape. In specimens that are immature or have recently 

 shed their eggs or sperm, the organs are quite inconspicuous and need 

 not be noted for the present. The paired digestive glands, which are 

 a yellowish-green color in a freshly killed specimen, will be easily 

 recognized, although in specimens with large ovaries they may be 

 crowded out of sight and can be found only by pressing aside the 

 latter organs. 



(b) Cut off the tops of the gills, sever the extensor muscles of the 

 abdomen at the level of the heart, cut back along each side of the 

 abdomen as far as the terminal portion, or telson, and remove the 

 dorsal skeleton of this region. The abdominal extensors will be found 

 continued as two thin bands of muscle lying close under the skeleton. 

 They may have been removed with the skeleton. The intestine lies 

 in the abdominal region along the mid-line. Beneath and to the sides 

 of the intestine are masses of muscle, which are the flexor muscles of 

 the abdomen. Compare the bulk of these flexors with that of the 

 extensors. Why should there be such a difference in the size and hence 

 the power of these muscles? You will perhaps make out, lying on top 

 of the intestine, a very small transparent thread that is colored in 

 injected specimens, the unpaired posterior aorta (dorsal abdominal 

 artery), which gives off branches to the muscles, intestine, and to all 

 the abdominal appendages except the most anterior pair. At the an- 

 terior end of the abdomen the median portion of the reproductive 

 organs may be found; or, if these are immature, the posterior ends of 

 the digestive glands. Review all the foregoing items of internal struc- 

 ture, noting the appearance of the digestive and circulatory systems 

 when viewed laterally (Fig. 66). Make an outline (X 2 or 3) of the 

 cephalothorax and abdomen. Put into this the organs as they lie in 

 place. 



(c) The heart, posterior aorta, and branchial vessels have been 

 identified. Other parts of the circulatory system (Fig. 66) which can 

 be seen by pushing the organs aside without further dissection in in- 

 jected specimens include: the median anterior aorta (ophthalmic 

 artery) to the head; paired lateral cephalic arteries (antennary ar- 

 teries) to the stomach, green glands, and anterior muscles; paired 

 lateral visceral arteries (hepatic arteries) to the gonads, anterior part 



