SALPA IN RELATION TO EVOLUTION OF LIFE. 149 



minifera exhibit indications of secondary adaptation, and they have 

 undoubtedly been modified by competition with improved organisms 

 from the bottom. 



All the metazoa have pelagic larvae, or else larval or embryonic 

 stages, which must be regarded as the degenerated vestiges of a pel- 

 agic habit ; but in most cases these larvae have been so much changed 

 by the accelerated development of adult features, or by the acqui- 

 sition of habits or structures to fit them for the conditions of modern 

 pelagic life, that w^e can deduce little more from them than the 

 former existence of pelagic ancestors. When a pelagic larva is still 

 represented by a modern pelagic adult of minute size and simple 

 structure, as the tadpole larva of ascidians is represented by Appen- 

 dicularia, w^e may be confident that it is a pelagic production, and 

 that it existed in the primitive pelagic fauna, although this view is 

 directly opposed to accepted dogmas regarding the origin of the 

 Chordata. When all the members of a great group have a definite 

 pelagic larval stage which adheres to the same plan of structure in 

 all of them, we may be pretty confident that this larva is the repre- 

 sentative of a primitive pelagic adult animal, even if this ancestor 

 has now no unmodified descendants. 



To my mind the best example of the retention, by all the mem- 

 bers of a great group, of a larval stage which represents an extinct 

 primitively pelagic ancestor is to be found in the crustacean nauplius, 

 and notwithstanding the popular verdict against it, I do not hesitate 

 to regard the nauplius as a pure pelagic product, and to include it 

 in the primitive pelagic fauna, although I shall discuss this question 

 further on. In cases like that of the echiuoderms, where the pel- 

 agic larvae of the various classes and orders are very different from 

 each other in the details of their organization, we are hardly safe 

 in assuming more than the primitive existence of an unknown pel- 

 agic organism, from which they have been derived. This is true 

 to even a greater degree of the trochic larvae of annelids, molluscs, 

 etc., but while there is little ground for regarding the forms of these 

 modern larv» as ancestral, we must regard their pelagic habit as an 

 inheritance from unknown ancestors in the primitive pelagic fauna, 

 in which we must therefore include representatives of such larvae 

 as the molluscan veliger, the nemertain pilidium, the actinotrocha 

 of phoronis, the brachiopod larva, the ccelenterate plauula, and so 



