LARVAL FORMS : THEIR NATURE, ORIGIN AND AFFINITIES. 397 
symmetrical Coelentera we find very commonly a series of 
lobes of the body-wall or tentacles produced equally — with 
Fig. 16. — Two Stages in the development of Tornaria. (After Metschnikoflf.) 
The black lines represent the ciliated bands, m. mouth ; an. anus ; 
br. branchial cleft ; ht. heart ; c. body-cavity between splanchnic and 
somatic mesoblast layers ; w. so-called water-vascular vesicle ; v. cir- 
cular blood-vessel. 
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 Coelentera, 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 ob- 
liquely 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 Prostomiunij and all the organisms which thus 
develop a new main axis, oblique to the old main axis, may 
be called prostomiate.” 
It will be seen from this quotation that the aboral part of 
the body is supposed to elongate to form the trunk, while the 
prseoral region is derived from one of the tentacles. 
VOL. XX. NEW SER. D D 
