540 ADAM SEDGWICK. 



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

 which have nothing to do with the coelom. 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 coelom 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 — coelom 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. 



List of Papers referred to. 



1. Balfour, F. M. — ' A Treatise on Comparative Embryology,' vol. i, Loudon, 



1885 (2ud ed.). 



2. Balfour, F. M. — 'A Treatise on Comparative Embryology,' vol. ii, 



London, 1881. 



3. Balfour, F. M. — " Notes on the Development of the Araneina," * Quart. 



Journ. of Micr. Sci.,' vol. xx. 



4. Balfour, F. M. — "Anatomy and Development of Peripatus capeu- 



sis," 'Quart. Journ. of Micro. Sci.,' vol. xxiii. 



4«. Bourne, A. G. — " Contributions to the Anatomy of the Ilirudinea," 

 ' Quart. Journ. of Micro. Sci.,' vol. xxiv. 



