128 



BILATERALITY. 



among the lower animals is much more apparent than in the 

 higher. 



Such an arrangement of the organs of the body is called 

 Bilaterality ; and, as you see by these diagrams, (figs. 55 to 64,) 

 bilaterality is the basis upon which the animal structure is 

 erected ; and whatever modification there may be of this feat- 

 ure, this type of form, such a modification is subordinate to 

 the type. 



Perhaps some of you will call to mind the starfishes and sea- 

 urchins, which you may have read about in books as being 

 formed upon a plan which is called radiate, like the spokes of a 

 wheel, or the divisions of an orange. It has been represented 

 that these so-called Radiates differ essentially from all other 

 animals, because their organs are not arranged upon the plan of 

 bilaterality ; and that whatever appearance of bilaterality for 

 its presence is admitted in a certain sense there is in them, is 

 of secondary importance. Now as the Radialists, as I may call 

 them, have admitted, nay, have even claimed, that there is the 

 appearance of bilaterality among Zoophytes (Radiates), let us 

 see what we can add to this acknowledgment. Let us refer for 

 a moment, without intending to anticipate what I may say 

 hereafter, to the earliest forms that have appeared on our globe; 

 and, to make the case the more decisive, to what were probably 

 the only representatives of their class at one time. 



This figure (fig. 66) represents the body 

 of one of the Crinoids, a Hemicosmites, which 

 lived in the earliest geological age. It belongs 

 to the same class as the trepangs, such as 

 Caudina (fig. 54) and the starfishes (chap. x. 

 figs. 109, 110) ; but, unlike them, it is attached 

 at the end (fig. 66, s) opposite the mouth (m) 

 to a stern. The principal feature is the snout- 

 like protrusion (m), at the end of which is 



m 



Fig. 66. 



the mouth. 



From our knowledge of the course of the intestine 



Fig. 66. Hemicosmites. Natural size. A fossil Encrinitc, without its stem. 

 TO, the proboscis ; a, the aperture to the posterior end of the intestine ; o, the 



