522 



TUNICATA. 



or the derivation of the anterior region of the body in the Tunicate 

 larva through fusion from a large number of trunk-metanieres. 



We have already mentioned (p. 367) that van Beneden and Julin 

 deny the homology of the branchial slits and the peribranchial or 

 atrial cavity in the Tunicates with those of other Chordata. Only 

 the two clefts which form first in the Tunicates are really to be 

 regarded as true gill-slits. This view results from the ascription by 

 these authors to the entoderm of a considerable part in the develop- 

 ment of the peribranchial sacs. In the same way, van Beneden 

 and Julin doubt the homology of the heart in the Tunicates with 

 the heart of the Vertebrata. We shall only be able to judge of this 

 last view, which indeed receives decided support from the absence of 

 the heart in the Am.phin.nis, when the way in which this organ arises 

 in the Tunicates is fully established. While Seeliger, like van 

 Beneden and Julin, derives the pericardial sac in the Ascidian 

 larva from the entoderm, most of the statements of other writers 

 seem to render its mesodermal origin probable. We must, however, 

 constantly bear in mind that an actual endocardium is altogether 

 wanting in the heart of the Tunicata. 



This view, shared by many of the more recent writers (Balfour,, 

 van Beneden and Julin, Hatschek) that the Tunicates and the 

 Cephalochorda, to which the Vertebrates are allied, represent distinct 

 branches of the Chordate type connected together only at their roots, 

 is opposed to that of Dohrn (Nos. 15-19), who regards the Tuni- 

 cates as degenerate fish. The ( Jyclostoma and Amphioxus are by this 

 author thought to represent distinct stages in the series of degenera- 

 tive processes through which the organisation of the Tunicates is to 

 be derived from that of the fishes. This view rests principally upon 

 the proof which Dohrn attempted to establish that the hypobranchial 

 groove (endostyle) as well as the peripharyngeal ciliated bands of the 

 Tunicates, the homologue of which was discovered by Schneider in 

 Ammocoetes, are to be regarded as transformed gill-clefts, and the 

 thyroid-gland, the homology of which with the hypobranchial groove 

 had been maintained by W. Muller was said to represent a branchial 

 sac lying between the spiracle and the first branchial cleft, while the 

 ciliated arch is the homologue of the spiracular cleft (the pseudo- 

 branch of the Teleosteans).* The endostyle and the ciliated arch 



* [Dohbn stands aloiie in bis belief that the pseudobranch of the Teleosteans 

 is formed from the anterior wall of the original spiracular cleft and that, by 

 the later suppression of the cleft, the pseudobranch conies to lie in the first 

 branchial cleft. Most vertebrate morphologists regard this pseudobranch as 



