402 



THE CRANIATES AND THE ACRANIATES. 



a too literal interpretation of the phenomenon of embryonic growth in terms of 

 adult coelenterates. There is little or no foundation for the prevalent assumption 

 that the blastopore elongates, closes in the middle, leaving a mouth at one end 

 and an anus at the other. On the contrary, segmented animals elongate primarily 

 by a localized apical growth, never by stretching a gastrula lengthwise. Neither 

 the mesoderm nor the notochord ever helped form the walls of a primitive, func- 

 tional, alimentary canal, with the mesoccele opening into the enteroccele. 



The principal source of confusion in the interpretation of these fundamental 

 processes has been the failure to recognize the difference between a true gastrula 

 and a mesentoccele, between a trochosphere and a naupula, neurostoma and 

 hasmostoma, or the haemal and neural surfaces; and in assigning a fictitious and 

 artificial significance to the so-called " archenteron" and "ccelomic pouches." 

 We base our conclusions on the following assumptions, that we regard as 

 axiomatic. 



FIG. 267. A, B, C, Diagrams of an annelid larva in the trochosphere, or coelentreate stages, showing the rela- 

 tion's of the gastrula, blastopore, mouth, and anus to each other; also the origin of the trunk, as an outgrowth 

 from the primitive head; D, E, F, same of a molluscan larva. 



i. The fixed point in all morphological problems is the central nervous 

 system. When it is located, and its direction of prolongation determined, we may 

 identify the six sides of any bilaterally symmetrical, acrogenous animal, and approx- 

 imately locate the characteristic organs of each side. 2. In all bilaterally sym- 

 metrical animals, the primitive mouth, or neurostoma, and the neuron, or axial 

 cords of the central nervous system, are always laid down on the same side of the 

 body, or egg, i.e., the neural surface. 3. The neural surface increases in length 

 primarily by apical growth at the anal or posterior end of the principal axis. 4. 

 The right and left sides of the body are formed as lateral outgrowths from the 

 principal axis, the growth and differentiation being in a neuro-haemal direction. 

 5. The primitive mouth, or neurostoma, always lies between the anterior ends of 

 the lateral cords. 6. The anal or caudal end of the body is always the youngest. 



