GENERAL CONSIDERATIONS ON THE TUNICATES. 523 



would thus be met with in the Tunicates in a secondarily modified 

 form. This view rests especially on observations of the development 

 of the homologous structures in Ammocoetes. 



Even though it appears to us that Dohrn carries these homologies 

 too far, since we are uot inclined to assume direct genetic relations 

 between the Fishes and the Tunicata, and still less to regard the two 

 groups as independent branches derived from a common, primitive 

 racial form (Protochordata), we consider that it entirely justifies the 

 conviction that the Tunicates cannot be utilised to bridge over the 

 gap existing between the Chordata and the other branches of the 

 animal kingdom. Haeckel and Gegenbaur have been specially 

 prominent in their advocacy of this view. The hypothetical primitive 

 form of the Tunicates is represented as a typical Chordate with all 

 the features generally ascribed to that type. But no characteristics 

 are to be found either in the anatomy or the ontogeny of the Tunicata 

 which ally them directly to any one branch of the Invertebrata. The 

 Tunicates appear to us no more nearly related to the Invertebrata 

 than arc Ampkioxus or the Vertebrata. The specially striking- 

 features in the Tunicates, viz., the absence of segmentation, of the 

 coelom and the nephridia, the occurrence of asexual reproduction are 

 all characters which we cannot regard as primitive. They have been 

 newly acquired in connection with the attached manner of life. The 

 way in which we have to reconstruct for ourselves the primitive 

 Chordata (the common hypothetical racial form of the Tunicata, the 

 Cephalochorda and the Vertebrata) can only he discovered through 

 careful comparison of the ontogeny and anatomy of these three 

 -roups, and in this we are convinced that the chief stress must be 

 laid on Amphioxus. Such a reconstruction is at present specially 

 difficult, indeed is rendered almost impossible by the fact that our 

 knowledge is as yet too fragmentary to enable us to establish exactly 

 the homologies of the different organs in the three groups of the 

 Chordata. In illustration of this it may be mentioned that our 

 knowledge of the origin of the peribranchial cavities in the Tunicates 

 is still incomplete, and the question as to the homology of the ciliated 

 pit in the Tunicates with the hypophysis cerebri and other problems 

 are as yet also insoluble. 



a development of the anterior wall »t the first branchial cleft in no way con- 

 nected with the spiracular deft or with the pseudobranch of Elasmobranchs. 

 Consequently, if there is any truth in the comparison given above it would be 

 more in accordance with the generally accepted homologies to read pseudo- 

 branchiae of Elasmobrauchs for pseudobranchiae of Teleosteans.— Ed.J 



