124 SUMMARY. 



(c) the lateral compartinents containing the nerve-cords and 

 salivary glands, and (d) the portion in the appendage. 



Of these, without going into details, it may be said that a 

 arises as a space between the ectoderm and the endoderm, b, c, 

 and d as spaces in the thickened somatic walls of the somites. 

 The spaces are in communication with each other. 



The heart arises as a part of a which becomes separated 

 from the rest. Posteriorly it acquires paired openings into 

 the pericardium. It thus appears that the heart and various 

 divisions of the body cavity of the adult form a series of spaces 

 which have nothing to do with the ccelom. They all com- 

 municate with each other and seem to form a series of enor- 

 mously dilated vascular trunks, of which the heart is the 

 narrowest and alone possesses the property of rhythmically 

 contracting. 



To sum up it appears that the ccelom in Peripatus is an 

 inconspicuous structure in the adult, and has no connection 

 with the body cavity ; while, on the other hand, the spaces of 

 the vascular system are but little subdivided, and form the 

 heart and various divisions of the adult body cavity. 



If these results are applicable to the Arthropoda generally, 

 and there is no reason, from the similarity of the adult anatomy, 

 to doubt that they will be found to be so, the following mor- 

 phological features may be added to those generally stated as 

 appertaining to the group — ccelom inconspicuous, body 

 cavity consisting entirely of vascular spaces. 



In Vertebrates and most Annelids, on the other hand, the 

 parts in question are arranged as follows: — Body cavity 

 entirely coelomic; vascular spaces broken up into 

 a complicated system of channels (arteries, veins, 

 capillaries). 



In most Molluscs, finally, the pericardium alone is coelomic; 

 the vascular spaces being represented by the heart and the 

 more or less complicated system of spaces in the body. 



