108 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



dence in general form between Ascidian larvae and lowly vertebrates, 

 due he believed to a similar mode of life, i.e. free-swimming. He thus 

 considered the adult as the true or fundamental form of the Tunicate 

 group, and the larvae not as a reminiscence of ancestral freedom, but 

 as a larval form that for its special and temporary needs, assumed 

 through like requirements of mechanical stresses, due to the adoption 

 of a similar mode of progression, swimming, a form akin to that of 

 a lowly vertebrate. 



If we want to go astray in reasoning, it is often easy enough 

 with the exercise of a little ingenuity or else of some obtuseness to 

 sink deep in the mire of false deduction. Anyone not thoroughly 

 master of an intricate subject may easily mistake cause for effect, 

 and conversely, and I remember once when considering this Ascidian 

 problem, thinking what a grand opportunity exists in the life-history 

 of Appendicularians for such an one to go astray. Among these tiny 

 tailed swimmers, the habit exists, at certain times, of forming a 

 gelatinous envelope in which the animal lies enveloped for a short 

 time — the " Haus " this has been termed ; and it is obviously 

 homologous ,to the test of the fixed Ascidians. Now a very 

 probable line of descent of the latter is from Appendicularians 

 that have taken this " haus " stage as a permanent adult condition 

 and so have deserted the free-swimming life— evidence being that 

 the fixed Ascidians recapitulate very closely, in their larval condition, 

 the general form, together with many of the essentially characteristic 

 internal features of adult Appendicularians. But the student not 

 thoroughly on his guard against the pitfalls of evolutionary reasoning, 

 might easily deduce that the Appendicularians are arrested larvae 

 of fixed Ascidians ; that is, that certain larvae instead of performing 

 the full life-history of their race, retained the tailed form all their 

 adult life, and that the occasional formation of the " haus " is the 

 atavistic recollection of the fixed Ascidian condition ! ! 



In reality such conclusion is easily refuted, and there is really 

 no reasonable doubt that the tadpole larvae show the ancestral form 

 fairly clearly. To mention one argument only, why should they have 

 a nervous system infinitely superior to that of the adult, formed too 

 on a plan entirely different from any to be found among inver- 

 tebrates, but characteristic of vertebrates ? 



Correction. -On page 18, line 6, read " ovary " instead of " ovum." 



