G2 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



ostia, provided with lip-like valves, which act so as to allow the blood 

 only to enter the organ. 



From the heart, as central organ of the circulation, well denned 

 canals, the blood vessels, are then developed, which in the Invertebrata 

 may alternate with lacunae not provided with walls. In the simplest 

 cases it is only the tracts along which the blood travels from the 

 heart which are provided with independent walls, and developed into 

 blood vessels (marine Copepoda, Calanella, fig. 53). At a higher 

 stage of development not only do these efferent vessels acquire a 

 more complicated structure, but a part of the lacuna-system, especially 

 in the neighbourhood of the heart, acquires a membranous invest- 

 ment, and gives rise to vessels which carry the blood back to the 



Accl 



FIG. 54. Heart and blood vessels and gills of the crayfish. C, heart, in a blood sinus ; with 

 Ps several pairs of ostia; Ac, cephalic aorta; A.ab, abdominal aorta; As, sternal 

 artery. 



pericardial sinus, from which it passes through the venous ostia into 

 the heart (Scorpions, Decapods) (fig. 54). 



In other cases (Molluscs) the blood flows directly from the afferent 

 vessels into the heart, the walls of the vessel being directly continuous 

 with the walls of the heart. The heart in such cases consists of two 

 chambers, the one known as auricle serves for the reception of the 

 returning blood, the other known as ventricle for its propulsion 

 (fig. 55). 



The vessels passing from the ventricle and carrying the blood from 

 the heart are called arteries ; those returning the blood to it are 

 called veins, and, in the higher animals, are distinguished from the 

 arteries by their thinner walls. Between the ends of the arteries 

 and the beginning of the veins the body cavity intervenes either as 



