THE CRAYFISH 



331 



merely spaces through which the blood flows in the course of its cir- 

 culation. To such a system of cavities the term hoemocoele, or 

 blood cavity, may be collectively applied. The term ccdome, it 

 will be recalled, is applied to a cavity of more definite limits and 

 lined with a peritoneal epithelium as in the Annulata and the 

 Vertebrata. If there is anything in the crayfish that has been 

 derived from a true coelome in the animal's ancestors, it is probably 

 the cavity within the reproductive organs. 



When the crayfish is viewed laterally (Fig. 159), the heart is 

 5een in the dorsal region of the thorax, in a cavity that resembles 



Fig. 159. — Digestive and arterial systems of the crayfish. 



Instead of being continuous with veins, through capillaries as in a vertebrate animal 

 (c/. Fig. 60, p. 104), the blood of the crayfish passes from the ends of the arteries into spaces 

 between the internal organs, through which it flows slowly back to the pericardium. Such 

 spaces are called hsemocceles, because they thus contain blood. The circulation of the cray- 

 fish is representative of what occurs in the Arthropoda. Blood from a pericardial sinus 

 enters a dorsally placed heart through openings in its walls called ostia. From the heart it 

 passes by arteries, that may be well developed as in the Tayfish or confist of but a single 

 aorta (cf. Figs. 178 and 179, p. 355) and is thence distributed to the body. The presence 

 of ostia in the wall of the heart is a feature that is unique for the Phylum Arthropoda. 



a.a., antennary artery; d.a.a., dorsal abdominal artery; g.gl., green gland; h, heart; 

 md, mandible; o.a., ophthalmic artery; s.a., sternal artery; v.a.a., ventral abdominal 

 artery. (Redrawn with modifications from Howes, "Atlas of Zootomy," copyright, 1902, 

 by Macmillan and Co., Ltd., printed by permission.) 



a pericardium, although it is only one of the larger sinuses of the 

 hsemocoele and hence must be called a pericardial sinus. Arteries 

 extend from the heart to all parts of the body, as the figure shows. 

 The digestive system is as shown. The brain lies anteriorly near 

 the eyes and antennae and there are connectives around the 

 esophagus to the nerve cords on the ventral side of the body. The 

 muscular system is complex, particularly in the abdomen where 

 there is a large ventral mass of flexor muscles which bends 

 this part of the body in the powerful swimming strokes, and a 

 much smaller mass of extensor muscles on the dorsal side. The 

 various movements of the appendages are made by similarly 



