158 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



The modern pelagic protophytes have probably retained nearly their 

 ancient form, but the modern radiolarians and pelagic foraminifera 

 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 pelagic 

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

 accelerated development of adult features, or by the acquisition of habits 

 or structures to fit them for the conditions of modern pelagic life, that 

 we 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 Appendicularia, we 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 members of 

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

 pelagic ancestor is to be found in the Crustacea nauplius, and notwith- 

 standing 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 echinoderms, where the pelagic 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 primi- 

 tive existence of an unknown pelagic 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 larvae 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 nemertian pilidium, the acti- 

 notrocha of phoronis, the brachiopod larva, the coelenterate planula, and 

 so forth, although we are quite unable to say how many independent 

 starting-points these various metozoan lines had in the primitive pelagic 



