403 COMPARATIVE ANATOMY. 



A phenomenon worthy of the greatest attention appears in the 

 Acopa in the disposition of the branchial slits, and is in the Salpas 

 alone obliterated by the peculiarities of their organisation. This 

 is the disposition of the slits in the manner of metameric 

 structures. In Doliolum they form two rows of transverse clefts, 

 and in Pyrosoma and Ascidians their transverse arrangement is also 

 perceptible, although in the latter several or many slits occur in a 

 cross series. Although this disposition is exhibited merely in a part 

 of the gut, yet we are able to recognise in it a condition which 

 supports the interpretation of it as an instance of metamerism. 

 Metamerism exhibits itself here in fact without the participation of 

 the entire organism, and it is not difficult to understand how, under 

 certain conditions, other parts of the body might take part in it. 



§ 311. 



The close relationship among themselves of all divisions of the 

 Tunicata is further exhibited in the existence of peculiar organs 

 belonging to the respiratory chamber which are connected with the 

 nutritive functions of the animal. These organs are the ventral 

 groove and the ciliated tracts. The ventral groove (hypo- 

 branchial groove) (Fig. 211, Bn), also called En do style, is a groove 

 lying in the ventral median line of the branchial chamber's wall, 

 possessing up-standing margins (*) (ventral folds), and terminating 



B 



Fig. 211. Diagram to exhibit the relations of the respiratory chamber to the ventral 

 groove. A in Balanoglossus. Bin Tunicata. r Respiratory chamber, n Ventral 



groove. * Ventral folds. 



at its anterior as well as at its posterior extremity in a blind 

 dilatation. 



The sides of the groove, which is at first in the Salpas very broad, 

 but later becomes narrow as in the other Tunicata, do not sink 

 equally below the surface in all parts, but form in various regions a 

 variety of prominences which may be described as longitudinal 

 ridges parallel with the groove. Between these are more or less 

 deeply-cut fissures, so that the contour of the groove in cross-section 

 is described by a much contorted line. The epithelium of the 

 respiratory chamber exhibits even at the free edge of the groove 



