ix ENTEROPNEUSTA PHYLOGENY 593 



supported chiefly upon a comparison of larval forms. The following characteristics 

 of the Trochophoran and the Tornarian larvee have been pointed out. 



The neural plate, the apical sensory organs, and the muscles which become 

 attached to the neural plate correspond in the two. The divisions of the intestine 

 also agree, that is, if the fore-gut of the Tornaria is an ectodermal stomodseum, 

 and the hind-gut a proctodseum. The two pairs of ccelomic sacs are comparable 

 with the two anterior pairs of mesoderm vesicles. 



The comparison of the ciliated bands presents difficulties. Three ciliated rings, 

 a preoral (surrounding the apical area with the neural plate), a postoral, and a 

 preanal, are typically ascribed to Trochophora, the last of which is supposed to 

 correspond with the principal ciliated ring of Tornaria, while the preoral and 

 the postoral rings of Trochophora are wanting in Tornaria. And it is argued 

 that the absence of these rings in Tornaria has led to the specialisation of the 

 preanal as the principal ciliated ring. 



On the other hand the preoral ciliated ring of Tornaria cannot be compared 

 with the preoral ring of Trochophora, because it does not surround the apical 

 plate. This latter lies, on the contrary, outside the frontal area, at the opposite 

 end of the oral region to the mouth ; the preoral ring passes in front of it. 



Compared with the principal ciliated ring, the pre- and postoral rings are, 

 perhaps, of small morphological significance, since they are wanting in the non- 

 pelagic larva of Balanoglossus Kowalevskii, whereas the principal ring occurs 

 in it. 



In a comparison of the Tornaria with the Trochophora larva, the great import- 

 ance of the following differences must not be overlooked. 



1. Trochophora possesses typically one pair of primitive kidneys, which are 

 wanting in Tornaria. 



2. Tornaria has a preoral coelom, which is absent in the Trochophora larva. 



If now we turn to the organisation of the adult animals for light as to this 

 question of the relationship between the Annelida and the Enteropneusta, we find 

 immediately that insurmountable difficulties stand in the way of any close com- 

 parison. Only in the blood vascular system is any fundamental agreement found. 

 The blood in the dorsal and ventral longitudinal vessels has the same course in 

 both groups. But, on the other hand, a comparison of the nervous system of the 

 Enteropneusta with that of the Annelida encounters difficulties similar to those 

 found in comparing it with that of the Vertebrata. The gills in the two groups 

 are altogether heterogeneous. The typical Annelidan kidneys are wanting in the 

 Enteropneusta, for the collar pores, which are probably of ectoblastic origin, can 

 hardly be regarded as a pair of nephridia. 



Thus the relationship of the Enteropneusta to the Annelida appears at the 

 best to be extremely distant. 



D. The relation of the Enteropneusta to the Echinodermata. This relation- 

 ship is claimed on the ground of the agreement existing between the larval forms. 

 The similarity of the Tornaria, especially with the Bipinnaria larva of the 

 Asteroidca, is, indeed, so striking that the first observer of Tornaria expected 

 for certain that it would develop into an Echinoderm. 



A closer comparison yields the following results : 



1. If we place the Tornaria and the Bipinnaria similarly with regard to the 

 position of the mouth and the anus, a striking agreement in the conformation 

 of the regions of the body and in the ciliated rings bordering them is observed 

 (Fig. 468). In both we find a separate preoral ciliated ring in the same position, 

 bordering a preoral area. In both we can distinguish, lying behind this, a 

 deepened oral area with the mouth in its ventral centre. The postoral ciliated 

 ring of the Tornaria corresponds with the large circumoral ring of the Bipinnaria. 

 VOL. II 2 Q 



