62 MARINE BIOLOGICAL LABORATORY. 



described further on. The newly formed trunk ulti- 

 mately becomes so large that the original substance of 

 the trochophore forms the head only,^ which is in Poly- 

 gordins scarcely larger than one of the trunk-segments. 

 The cilia of the prototroch disappear, the animal sinks 

 to the bottom, burrows into the sand, and assumes the 

 adult condition. 



Let us examine the significance of these facts. It 

 appears that the head is the oldest part of the body, and 

 that in a certain sense the trunk is its offspring, — as 

 a branch is an offshoot from a tree. In other words, 

 the Qgg develops into what we may regard as a free- 

 swimming head, and this after a while buds off the 

 body — an afterthought, as it were. Observe now the 

 genealogical question that is at once raised. If this 

 mode of development be in any manner a repetition or 

 representation of the ancestral development, then the 

 ancestors of the annelids, and of all the higher metam- 

 eric types, are represented to-day in the head. And 

 the head must, therefore, be historically, as well as em- 

 bryologically, the oldest part of the body, and the trunk 

 is a later acquisition.^ 



We may go farther than this. The trunk, I have 



^ This statement demands some qualification, since the extreme lower 

 pole of the larva, bearing the anus, is carried down with the growth of 

 the trunk, and remains as the so-called telson of the adult. It is, how- 

 ever, unnecessary to complicate the discussion by bringing in this fact. 



2 The " head " of arthropods and vertebrates probably represents the 

 prostomium plus a certain number of trunk somites, closely united and 

 devoted to special functions. Indeed, these somites have so far usurped 

 the functions of the original prostomium that it is doubtful whether this 

 can any longer be distinguished in the arthropod or vertebrate embryo. 

 This, however, does not affect the essence of the historical question under 

 consideration. 



