SECTION D.-ZOOLOGY. 



THE ORIGIN AND EVOLUTION OF 

 LARVAL FORMS. 



ADDRESS BY 



PROF. WALTER GARSTANG, M.A., D.So., 



PRESIDENT OF THE SECTION. 



The transformations, or metamorphoses, of animals have always provided 

 one of the most fascinating chapters of Descriptive Zoology. Their 

 significance in relation to the doctrine of Evolution was a subject of 

 animated debate by previous generations of zoologists, and figured largely 

 in several Presidential Addresses to Section D, notably in those of the 

 late Prof. Milnes Marshall, in relation to the theory of Recapitulation, at 

 the Leeds Meeting in 1890, and of the late Prof. Miall, from the standpoint 

 of Adaptation, at the Toronto Meeting in 1897. The conclusions arrived 

 at by these two distinguished predecessors of mine were by no means 

 concordant, and I hope I am not wrong in thinking the time ripe for 

 reopening the subject. I propose, however, to take it from a third 

 standpoint, distinct from theirs, yet related, which I may broadly define 

 as the part played by larval forms in the course of evolution. 



If we take any large class of marine Invertebrates the members of 

 which can be seen to have made substantial progress along one or more 

 lines of descent, a comparison of their larval forms shows that on the 

 whole a larval evolution has taken place more or less parallel to that of 

 the adult evolution, but subject to conspicuous deviations. Primitive 

 types of larvae are limited to the lower or more primitive sections of the 

 class, and secondary larval characters become more and more pronounced 

 in the higher and more recent members. In this general statement I am 

 thinking of classes like Mollusca and Crustacea, in which the metamorphosis 

 is gradual and continuous, and is not subject to sudden and radical changes 

 of plan, such as are exhibited for example by Echinoderms and Polyzoa. 



In Mollusca the primitive type of larva is obviously a Trochosphere, 

 closely resembling that of Annelids in its pear-shaped body, praeoral 

 ciliated ring or prototroch, apical tuft, and absence of special Molluscan 

 features such as shell and foot. It is found in each of the main sub-classes 

 of Mollusca, except the Cephalopoda, viz. in Chiton (Amphineura), Patella 

 and Acmcea (Gastropoda), Dentalium (Scaphopoda), and Nucula and 

 Yoldia (Lamellibranchia or Bivalvia). All these are genera which, either 

 in Mollusca as a whole, or in their respective sub-classes, retain a distinct 

 preponderance of archaic characters — Patella and Acmcea belonging to the 

 lowest section of Gastropoda (Zygobranchia, in spite of loss of the original 



