PRESIDENTIAL ADDRESS. 41 



of one part of the surface, sufficient to lead to its preservation and 

 further development. 



Of larval forms later than the gastrnla, the most important by far 

 is the Pilidium larva, from which it is possible, as Balfour has shown, 

 that the slightly later Echinoderm larva, as well as the widely spread 

 Trochosphere larva, may both be derived. Balfour concludes that the 

 larva forms of all Ccelomata, excluding the Crustacea and vertebrates, 

 may be derived from one common type, which is most nearly repre- 

 sented now by the Pilidium larva, and which ' was an organism some- 

 thing like a Medusa, with a radial symmetry.' The tendency of 

 recent phylogenetic speculations is to accept this in full, and to regard 

 as the ancestor of Turbellarians and of all higher forms, a jelly-fish or 

 ctenophoran, which in place of swimming freely has taken to crawling 

 on the sea bottom. 



Of the two groups excluded above, the Crustacea and the vertebrata, 

 the interest of the former centres in the much discussed problem of 

 the significance of the Nauplius larva. There is now a fairly general 

 agreement that the primitive Crustacea were types akin to the 

 phyllopods, i.e., forms with elongated and many-segmented bodies, 

 and a large number of pair of similar appendages. If this is correct, 

 then the explanation of the Nauplius stage must be afforded by the 

 phyllopods themselves, and it is no use looking beyond this group for 

 it. A Nauplius larva occurs in other Crustacea merely because they 

 have inherited from their phyllopod ancestors the tendency to develop 

 such a stage, and it is quite legitimate to hold that higher crustaceans 

 are descended from phyllopods, and that the Nauplius represents in 

 more or less modified form an earlier ancestor of the phyllopods 

 themselves. 



As to the Nauplius itself the first thing to note is that though an 

 early larval form it cannot be a very primitive form, for it is already 

 an unmistakable crustacean ; the absence of cilia, the formation of a 

 cuticular investment, the presence of jointed schizopodous limbs, 

 together with other anatomical characters, proving this point con- 

 clusively. It follows, therefore, either that the earlier and more 

 primitive stages are entirely omitted in the development of Crustacea, 

 or else that the Nauplius represents such an early ancestral stage 

 with crustacean characters, which properly belong to a later stage, 

 thrown back upon it and precociously developed. 



