280 ZOOLOGY. 



slit-like openings, with valves. Definite arterial vessels leave 

 the heart and pass to capillary regions and thereupon open 

 into irregular spaces in the tissues without definite walls 

 (lacunae}. The haemoccele is in reality an enlarged lacuna. 

 In insects there is an anterior artery only ; in spiders and crus- 

 tacea, posterior and lateral arteries also occur. The return of 

 the blood takes place through the irregular hsemoccele spaces 

 (lacunae). These become more definite in form as they near 

 the pericardial chamber, or as they approach the gills in 

 aquatic forms. One of the more important blood spaces is 

 the ventral, in which the nerve cord lies (Fig. 124, v.s.). The 

 blood corpuscles are colorless and amoeboid. The plasma may 

 be variously colored by pigments which seem to assist in the 

 work of respiration. 



316. Excretion. The importance of excretion increases 

 with the activity of animals. Except in Peripatus it is not 

 conclusive that any of the adult excretory organs in this 

 phylum are homologous with the segmental organs of Annu- 

 lata. In insects and spiders there are excretory tubules com- 

 municating with the hind gut. In the cray-fish and related 

 forms a pair of excretory glands " green glands " open at 

 the base of the antennae. It is of importance to remember 

 that the exoskeleton of the Arthropoda is an excretion, which 

 is incidentally protective and supportive. 



317. The Nervous System consists essentially of the same 

 parts as have been described for the annelids. It is, however, 

 on the whole, more fully developed. This development accords 

 with the differentiation which we have seen in the somites and 

 body regions. The brain and sub-cesophageal ganglia, for 

 example, have become more pronounced with the differentia- 

 tion of the head; accompanying the fusion of the body seg- 

 ments there is a massing of the corresponding ganglia; and 

 in general, everything considered, those ganglia are best de- 

 veloped which lie in the best developed somites. The concen- 

 tration of the ganglia of the ventral cord may continue until 



