LARVAL FORMS. 



309 



that the ventral and median position of the mouth in many Turbel- 

 laria is the primitive one. 



The above suggestion as to the mode of passage from the radial into the 

 bilateral form differs largely from 

 that usually held. Lankester 1 , for 

 instance, gives the following ac- 

 count of this passage : 



"It has been recognised by 

 various writers, but notably by 

 Gegenbaur and Haeckel, that a 

 condition of radiate symmetry 

 must have preceded the condition 

 of bilateral symmetry in animal 

 evolution. The Diblaatula may be 

 conceived to have been at first 

 absolutely spherical with spherical 

 symmetry. The establishment of 

 a mouth led necessarily to the 

 establishment of a structural axis 

 passing through the mouth, around FlG - 22 ?- LARVA OF ECHIUBUS. (After 



i i - j-i i i i Salensky.) 



winch axis the body was arranged 



,i ,. , mi w>. mouth; an. anus; sq. supra-oesoplia- 



with radial symmetry. This con- gea l ganglion (?). 



d it ion is more or less perfectly 



maintained by many Coelenterates, and is reassumed by degradation of 

 higher forms (Echinoclerms, some Cirrhipedes, some Tunicates). The next 

 step is the differentiation of an upper and a lower surface in 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 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 development of a region in front of, 

 and dorsal to the mouth, forming the Prosto- 

 itiium, was accomplished pari passu with the 

 development of bilateral symmetry. In the 

 radially symmetrical Ccelentera we find very 

 commonly a series of lobes of the body-wall 

 or tentacles produced equally with radial 

 symmetry, 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 Cce- 

 lentera, is attained by shifting what was the 

 main axis of the body so that it may be 



FIG. 228. DIAGRAM OF A 



LARVA OF THE PoiA'ZOA. 



m. mouth; an. anus; st- 

 stomach; s. ciliated disc. 



52, p. 140, as being intermediate between the Ctenophora and the Turbellaria. As 

 already mentioned, there does not appear to me to be sufficient evidence to prove that 

 this form is not merely a creeping Ctenophor. 



1 Quart. Jonrn. of Micr. Science, Vol. xvn. pp. 422-3. 



