284 ZOOLOGY 



322. Circulation. — The heart or pulsating organ when 

 present is dorsal and may be much elongated, with an enlarge- 

 ment in each somite. It lies in a membrane-bounded cavity 

 called the pericardial sinus (Fig. 126, ps.), which is a part of 

 the hsemoccele or secondary body cavity (§317). The blood 

 comes to the pericardial cavity and enters the heart by means 

 of 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 (lacuncB). 

 The hsemocoele is in reality an enlarged lacuna. In insects 

 there is an anterior artery only; in spiders and Crustacea, pos- 

 terior and lateral arteries also occur. The return of the blood 

 takes place through the irregular haemocoele 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. 126, v.s.). The blood corpuscles are color- 

 less and amoeboid. The plasma may be variously colored by 

 pigments which seem to assist in the work of respiration, as 

 haemoglobin does in vertebrates. 



3^3. 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 Annelida. In 

 insects and spiders there are excretory tubules communicating 

 with the hind gut. In the crayfish and related forms a pair 

 of excretory glands — "green glands" — open at the base of the 

 antennas. It is of importance to remember that the exoskeleton 

 of the Arthropoda is an excretion, which is incidentally protective 

 and supportive. 



324. The Nervous System consists essentially of the same 

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

 on the whole, more ftilly developed. This development accords 

 with the differentiation which we have seen in the somites and 

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

 example, have become more pronounced with the differentia- 

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



