134 CHANGES FROM STAGE G TO BIRTH. 



gular shape (PI. XII, fig. 13). The muscles, both longi- 

 tudinal and circular, are deposited outside the commissures 

 connecting the ventral nerve-cords. 



The muscles of the feet seem to be derived from the fibrous 

 plexus shown in PI. X, fig. 1, and are shown at a later stage 

 in Pi. XI, fig. 11). The origin of the transverse septa dividing 

 the body cavity into a central and two lateral compartments 

 has already been described (pp. 76 and 78, PL VI, fig. 9, Pi. 

 VII, figs. 21a, 24-, PI. VIII, fig. 39, &c., v. s.). They arise as 

 outgrowths of the ventral corners of the somites. 



The contractile tissue of the gut wall and internal organs 

 generally, is derived from the wandering cells, which them- 

 selves appear to be derived from the walls of the mesodermal 

 somites. 



The Body Cavity and Vascular System. 



As was first pointed out by Lankester, the Arthropoda are 

 distinguished from all other animals by the possession of paired 

 ostia, perforating the wall of the heart and putting its cavity 

 in communication with the pericardium. The pericardial 

 cavity of Arthropoda, therefore, contains blood, and in this 

 respect differs fundamentally from the similarly named cavity 

 in other animals. Not only does the pericardial division of 

 the body cavity contain blood, but the general body cavity, 

 and in Peripatus all the compartments of the latter are also 

 vascular tracts; and it is important to distinguish by a special 

 name this vascular type of body cavity from the non-vascular 

 or ccELOMic type which is found well developed in Annelida 

 and Vertebrata. The term " h^emoccele," which has been 

 suggested by Lankester for this purpose, seems a convenient 

 one, and I propose to adopt it. 



The development of the hsemocoele of Peripatus has been 

 already fully described on p. 82, et seq., and p. 93 ; 

 and I have but little to add to the account there given. It 

 is derived in part from a system of species developed within 

 the mesoderm, and in part from spaces arising between the 



