GENERAL CONSIDERATIONS. 579 



the Chordata and Balanoglossus. BATESON (No. 26) homologises the 

 dorsal nerve-strand in the collar region of Balanoglossus (the so-called 

 collar-cord) with the medullary tube of the Vertebrates. The anterior 

 intestinal diverticulum (the so-called proboscis-intestine) of Balano- 

 gloaxus ( Vol. i., Fig. 165, di, p. 375), according to him and to KOEHLER, 

 is the homologue of the notochord. The rudiment of the so-called 

 proboscidal coelom is homologised with the anterior unpaired entoderm- 

 diverticulum of Amphioxus (Fig. 285 B). The external aperture of 

 the left anterior entoderm-diverticulum of Amphioxus is assumed to 

 correspond to the proboscis-pore of Balanoglossus. A posterior fold 

 in the collar-region, called by BATESON the "operculum," is said to 

 correspond to the epipleura of Ampliiuxus. Finally, even GEGENBAUR 

 compared the ventral nutritive section of the pharynx in Balanoglossus 

 (Vol. i., Fig. 166, d, p. 377) to the endostyle of the Tunicates. 

 SPENGEL (No. 47) has pointed out the difficulties that stand in the 

 way of such a homology, and lays special stress on the great difference 

 in the position of the gills which, in Balanoglossus, are dorsal, and, in 

 Amphioxus, ventral the development of the blood-vascular system 

 and the genital organs in the two groups. The bare fact of the 

 presence of gills in Balanoglossus, indeed, and their remarkable and 

 detailed agreement in structure and arrangement with those of 

 Amphioxus (U-shaped form in consequence of the growth of tongue- 

 bars, chitinous skeletal structures in the form of prongs, presence of 

 synapticula) seem to us of such significance that we cannot avoid 

 the thought that we have, in Balanoglossus, the only living form of 

 Invertebrate which is closely related to the Chordata. But, if we 

 adopt this assumption, the Chordata naturally become somewhat 

 more remote from the Annelida, since Balanoglossus is only distantly 

 related to this last group. How far it is possible to remove the diffi- 

 culties which now lie in the way of establishing a stricter homology 

 between Balanoglossus and Amphioxus must be decided by further 

 research. 



The problem of the derivation of the Chordata is not solved by 

 assuming a relationship between them and the Enteropneusta, since 

 the latter themselves occupy an unusually isolated position. Only 

 through the structure of the Balanoglossus larva is an indication 

 given of remote connection with the Echinoderma. We must 

 resign ourselves to the thought that we are not at present in a 

 position to state from what primitive form the Chordata and, with 

 them, Balanoglossus are to be derived. The origin of the Vertebrates 

 is lost in the obscurity of forms unknown to us. 



