LARVAL FORMS. 



375 



tion of higher forms (Echinoderms, some Cirrhipedes, some Tunicates). 

 The next step is the differentiation of an upper and a lower surface in 



FIG. 227. LARVA OF ECHIURUS. (After Salensky.) 

 m. mouth ; an. anus ; sg. supra-cesophageal ganglion (?). 



relation to the horizontal position, with mouth placed anteriorly, assumed by 

 the organism in locomotion. With the differentiation of a superior and 

 inferior surface, a right and a left side, complementary one to the other, are 

 necessarily also differentiated. Thus the organism a/t 



becomes bilaterally symmetrical. The Ccelentera 

 are not wanting in indications of this bilateral 

 symmetry, but for all other higher groups of animals 

 it is a fundamental character. Probably the de- 

 velopment of a region in front of, and dorsal to the 

 mouth, forming the Prostomium, was accomplished 

 pari passu with the development of bilateral sym- 

 metry. In the radially symmetrical Ccelentera we 

 find very commonly a series of lobes of the body- 

 wall or tentacles produced equally with radial sym- 

 metry, that is to say all round the mouth, the 

 mouth terminating the main axis of the body that 

 is to say, the organism being 'telostomiate.' The 

 later fundamental form, common to all animals above the Ccelentera, is 

 attained by shifting what was the main axis of the body so that it may be 

 described now as the ' enteric ' axis ; whilst the new main axis, that parallel 

 with the plane of progression, passes through the dorsal region of the body 

 running obliquely in relation to the enteric axis. Only one lobe or outgrowth 

 of those radially disposed in the telostomiate organisms now persists. This 

 lobe lies dorsally to the mouth, and through it runs the new main axis. This 

 lobe is the Prostomium, and all the organisms which thus develop a new 

 main axis, oblique to the old main axis, may be called prostomiate." 



FIG. 228. DIAGRAM 

 OF A LARVA OF THE 



POLYZOA. 



m. mouth ; an. anus ; 

 st. stomach ; s. ciliated 

 disc. 



