592 COMPARATIVE ANATOMY CHAP. 



1. The great similarity of the Enteropneustan gills to those of Amphioxus iu 

 finer structure remains, but the detailed comparison of point with point makes a 

 real homology doubtful, and seems to oblige us to consider the resemblance as at 

 the most a very remarkable case of convergence. 



Further, the following considerations must also be taken into account. The 

 gills of Amphioxus arise ontogenetically as segmental structures, while those of 

 the Enteropneusta, although standing in a longitudinal row, belong to the unseg- 

 mented trunk. 



The gills of the Chordata receive their blood from the ventral vascular trunk, 

 those of the Enteropneusta from the dorsal trunk. 



Should it be proved that the larval oesophagus of the Enteropneusta proceeds 

 from an ectodermal invagination, and is a stomodseum, then the gills of the 

 Enteropneusta would lie in an ectodermal intestinal region, in contradistinction 

 to those of the Vertebrata, which belong to an endodermal intestinal region. 



2. The proboscidal diverticulum is a preoral outgrowth of the wall of the buccal 

 cavity, and is lined with epithelium. It does not thus agree with the tissue of 

 the noto-chord. It is further very questionable whether the buccal cavity, 

 and, with it, the diverticulum are endodermal formations. The proboscidal 

 diverticulum lies below the continuation of the dorsal blood vascular trunk (below 

 the central blood sinus of the proboscis) ; the chorda of Vertebrates, on the con- 

 trary, lies above the dorsal blood vascular trunk (aorta). K"o homology between 

 the two is possible. 



3. The proboscidal skeleton, as a thickened limiting membrane, could at the 

 most be compared only with the inner cuticular sheath of the chorda. 



4. The body cavity is an enteroccel, in many different divisions of the animal 

 kingdom (either constantly or exceptionally) other than the Enteropneusta and the 

 Chordata. The ccelomic vesicles (mesoderm vesicles, primitive vertebrae) of the 

 Vertebrata show a segmental arrangement, corresponding with the metamerism of 

 the other organs, while in the Enteropneusta no such arrangement exists. 



5. The collar cord of the Enteropueusta is only the anterior continuation of the 

 dorsal nerve cord of the trunk. It does not sink below the surface until all its 

 parts are formed. The corresponding ventral nerve cord of the Enteropneusta does 

 not exist anywhere in the Chordata. 



The following further points must be emphasised. 



The gonads of Amphioxus arise segmentally from the endothelium of the body 

 cavity, while the rows of gonads in the Enteropneusta lie in an unsegmeuted 

 region. The first origin of the gonads of the Enteropneusta is, indeed, unknown, 

 but their rudiments are found in the blastoccel very early. The manner in which 

 the genital products are ejected iu the two groups is altogether different. 



In the Chordata, the blood in the dorsal vessel flows from before backward, in 

 the ventral from behind forward ; the reverse is the case in the Enteropneusta. 



A comparison of the two collar pores in the Enteropneusta with the most 

 anterior pair of nephridia in Amphioxus could only be of value were the develop- 

 ment of these organs known. In all probability the former are of ectodermal and 

 the latter of mesodermal origin. 



There is nothing we know of in the Chordata comparable with the Tornaria 

 larva. 



These considerations render any relationship between the Enteropneusta and 

 the Chordata, at least at present, highly improbable. 



B. The relationship between the Enteropneusta and the Nemertina is so very 

 problematical that it cannot here be discussed. 



C. Eolation of the Enteropneusta with the Annelida. The attempt to bring 

 the Enteropneusta and the Annelida into even a distant genetic relationship is 



