* 



GENERAL CONSIDERATIONS ON THE TUNICATES. 519 



tion of the dorsal region found in the Ascidians and the approxima- 

 tion of the branchial and atrial apertures thus brought ahout is 

 Jfraced back to attached forms, the position of the anus in the 

 " Larvacea, which must evidently be regarded as primitive, would 

 indicate thai these animals are descendants of those hypothetical 

 Tunicate ancestors which still retained the original pelagic life. 

 <>n the other hand, we find in Appendicularia a series of unmistakable 

 degenerative phenomena tending t<> support the assumption that we 

 must regard the racial form of the Larvacea as an attached Tunicate. 

 It is evident that they must be considered as sexually mature larval 

 tonus, sexual maturity being shifted continually further back to 

 earlier stages so that, finally, the mature adult form no longer 

 developed. What form can we imagine this latter to have assumed I 

 Was it a free swimming Ascidian form intermediate between AmphioxUs 

 and the Ascidian larva, or an already attached Ascidian form ' The 

 last assumption seems the more probable. The appearance of the 

 cellulose mantle and hermaphroditism and the indistinctness of the 

 segmentation of the body must be regarded as features acquired as 

 a consequence of attached manner of life. As these characters are 

 found in the Larvacea, we are to a certain extent justified in con- 

 sidering them as the sexually mature larvae of an already attached 

 form of Tunicate.* In any case, however, all phylogenetic specula- 

 tions concerning the Tmiicates must rest upon careful consideration 

 of the structure of Appendicularia and the Ascidian larvae. 



Among the Ascidiacea the solitary forms are probably the more 

 primitive. The composite forms lead on to Pyrosoma, which may lie 

 regarded as a composite colony that has not become attached and 

 that is distinguished by a large common cloaca. In the interesting 

 family Coelocormidae we see the development of the whole colony in 

 a similar direction. Here also the colony is not attached but, on the 

 other hand, the internal cavity cannot be compared to the common 

 doaca of Pyrosoma (see Herdman, No. 24). Pyrosoma forms 

 a transition to the free-swimming Doliolidae. This was pointed out 

 by Huxley with reference to the structure of the gill and the 

 opposite position of the two apertures of the body (</. also Grobben. 

 No. 79). The Doliolidae (Cyclomyaria), among which Doliopsis 

 [Anchinia) exhibits the most primitive characters, must be regarded 

 as phylogenetically the oldest Thaliacea. The Salpidou (Hemimyaria) 

 must be considered as derived from them. We shall, therefore, have 



*Willkv (No. 54a) also has lately maintained that, the Larvacea are 

 degenerate forms. 



