516 TUNICATA. 



same generation. Todabo would, however, strictly speaking, not be justified 

 in assuming an asexual multiplication of the embryo in early stages of 

 embryonic development. 



Salensky (Nos. 74 and 102) traces back the alternation of generations of 

 the Tunicates to metamorphosis. The acquisition of the capacity for asexual 

 reproduction by the larvae of the Tunicates, or, in other words, the shifting 

 back of this capacity originally belonging to adult forms to larval stages, 

 made it possible to spread out over several generations the series of trans- 

 formations which constitute metamorphosis. In this way we can explain the 

 dimorphism of the generations. Salensky consequently regards the solitary 

 form as a larval stage and the individuals of the chain as the adults. We, 

 however, with Leuckart (No. 98), hold on the contrary that we are justified 

 in regarding the solitary form just as much as the individual of the chain as 

 a fully developed individual. 



III. General Considerations on the Tunicates. 



If we attempt to draw from the ontogeny of the Tunicates con- 

 clusions of a more general nature, we are at once struck by the 

 difference of opinion prevailing among the various investigators of 

 this subject. As an example of this, we may mention that the 

 central nervous system in the larvae of the Ascidiacea is traced back 

 to an ectodermal invagination ; in the buds of the composite Ascidians, 

 on the contrary, according to statements made by Kowalevsky and 

 recently confirmed by H.iort (No. 59), it is derived from the ento- 

 derm of the bud.* In the Cyathozooid of Pyrosoma, and probably 

 also in the first four Ascidiozooids, it is derived from the ectoderm, 

 whereas Seeligek, in all the later-developed Ascidiozooids of the 

 colony, as in the buds of the Salpidnc, traces it back to the meso- 

 derm. Similar uncertainty prevails with regard to the development 

 of the peribranchial cavities and the atrium. We are also in doubt 

 as to how r far the ectoderm and entoderm take part in their formation 

 in Ascidian larvae, but in the Cyathozooid of Pyrosoma, in any case, 

 they are derived exclusively from the ectoderm. Nearly all authors, 

 however, are agreed that the peribranchial sacs become abstricted 

 from the central entoderm-sac in the buds of the Ascidiacea. In the 

 buds of the Salpidae and in the later Ascidiozooids of Pyrosoma, 

 Seeligek derives the peribranchial tubes from the mesoderm. It is 

 difficult to decide how far this difference of opinion can be accounted 

 for by errors of observation or to what extent actual differences exist. 

 We, for instance, find it difficult to assume that the buds of Pyrosoma 

 develop in a totally different way from those of the nearly related 



* [See footnote, p. 466. — Ed.] 



