OF B1LATERALITY. 



249 



a worm. Place them side by side with this embryo (fig. 148) 

 of a genuine worm, and no one could say positively, if they were 

 new to him, which is not a worm. Until it passes beyond this 

 embryonic condition, it is oftentimes extremely difficult to deter- 

 mine in which of the five grand divisions an animal belongs. 



This difficulty arises from the fact that the predominant idea 

 in all animals is bilaterality ; and for the reason that during the 

 early stages of growth this idea is expressed with a degree of 

 exclusion which obscures the subordinate idea, which finally is 

 superimposed upon the predominant one. 



Let us follow out the development of this idea among the 

 examples we have here, and see how it addresses itself to our 

 eyes. In these two figures (figs. 

 142, 143) of young Trepangs, you 

 will notice that the arrangement of 

 the visible parts of the organization 

 is in reference to a line which may 

 be drawn lengthwise through the 

 middle of the body ; that is, the 

 organs are either immediately upon 

 the axial line, as in this figure, (fig. 

 142,) or situated symmetrically right aq j 

 and left of it, as represented here 

 (fig. 143). This feature is the most 

 prominent one ; and notwithstand- 

 ing there is the beginning of a lat- 

 eral repetition of parts in one of 

 the figures, (fig. 143,) it is not so 

 decided, nor so far developed, as to Fi s- 144 - 



Fig. 142. Holothuria tremula. Gunn. Natural size about -^ of an inch long. 

 A view of the back of a very young Trepang. n, the future mouth ; t, feelers 

 beginning to develop ; of, the madreporic body, lying beneath the skin. From 

 Koren and Danielssen. 



Fig. 143. The same as fig. 142, but a little older. A view of the lower or 

 ventral side, m, the mouth developing; t, fl, the incipient feelers; s, the first 

 pair of sucker-like feet. From Koren and Danielssen. 



Fig. 144. Holothuria tremula. Gunn. The same as figs. 142, 143, at a far 



