PROFESSOR LANKESTEB. 



ture and connective tissues continue to take origin by dela- 

 mination from ectodermal cells at later stages. 



In all the large groups of the animal series^ excepting the 

 Coelentera and Echinoderma, a very great diversity in this 

 matter prevails. 



III. Considerations relative to the Development 

 OF External Form. 



A. Radial and bilateral symmetry and telostomiate 

 and prostomiate conditions .—It has been recognised by 

 various writers, but notably by Gegenbaur and Haeckel, 

 that a condition of radiate symmetry must have pre- 

 ceded the condition of dilateral symmetry in animal evolu- 

 tion. The Diblastula may be conceived to have been at 

 first absolutely spherical with spherical symmetry. The 

 establishment of a mouth lead necessarily to the establish- 

 ment of a structural axis passing through the mouth, 

 around which axis the body was arranged with radial 

 symmetry. This condition is more or less perfectly main- 

 tained by many Ccelenterates, and is reassumed by degrada- 

 tion of higher forms (Echinoderms, 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 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 Coelentera 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 prostomium, was acccomplished 

 pari passu with the development of bilateral symmetry. In 

 the radially symmetrical Coelentera 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 Coelen- 

 tera, 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 telosto- 

 miate organisms now persists. This lobe lies dorsally to the 



