~_, ae eS S-_-”-~~~—"— 
39 
The lecturer next dealt with the biradiate animal, in which, 
ag seen in the sea-anemone, whilst the radial symmetry is well 
preserved, there is superadded to it a further change in the shape 
and arrangements of certain of the internal organs; he then 
described a higher form, that of bilateral symmetry ; the animal, 
as in a worm, a lobster, or a frog, is divided by a medial 
vertical plane into symmetrical right and left halves, while, 
furthermore, a distinction may be readily made between dorsal 
and vertical surfaces, and between anterior and posterior ends. 
This shape, in its earlier phases, is connected with the habit of 
crawling along the sea bottom. When once bilateral symmetry 
is established, the further modifications seen in the higher forms 
become comparatively easy to follow: it is characteristic of all 
the higher groups of animals, though it may be marked or 
modified by further development, as the twisting of the body of 
a snail, the asymmetrical form of the tail of the hermit crab, or 
the shifting of the eye in a sole. 
Speaking generally, the forms of the higher animals are 
derived from those of the lower bilaterally symmetrical worms, 
by exaggerating the differences between one part of the body and 
another already present in these latter. Thus the differences 
between the dorsal and central surfaces, or rather halves of the 
body, and between the anterior and posterior ends of the body, 
gradually become intensified, attaining their maximum in birds 
and mammals, the two highest groups of animals. 
In dealing with the sizes of animals, the lecturer pointed out 
that in the natural or wild state, the size of each kind of animal 
was fairly well defined, the limits of variability being much 
wider in the case of aquatic, and especially of marine, than of 
terrestrial animals. As regards the actual dimensions attained, 
the aquatic animals lead the way. Of all animals now existing, 
whales are incomparably the largest ; next to the aquatic come 
the terrestrial forms, with the elephant in the forefront ; while 
last and smallest of all come the erial, or flying animals. The 
actual size seems to be associated with the density of the 
medium in which the animal lives. In water, an animal has to 
support but a very small part of its weight by its own muscular 
effort. In flying animals every part of the body is considerably 
heavier than the air, and great muscular efforts are necessary to 
sustain the animal during flight. Large size or great weight of 
the body become therefore impossible. 
UY BAS 
