640 ZOOLOGY. 
tailed larval Ascidian presents features which apparently 
anticipate the state of things existing among the lower ver- 
tebrates, such as the lancelet. 
In the last-named animal the nervous cord has a dorsal 
position—.e., rests above the alimentary canal; but as yet 
no brain appears, only a very slight enlargement of the an- 
terior end of the nervous cord from which a few nervous 
threads are distributed to minute sense-organs in the head. 
In all the craniate Vertebrates, from the lamprey upward, 
the brain is a series of close-set ganglia, having a definite 
site, enclosed by a skull or brain-box, and with definite re- 
lations to the sense-organs. Attention has already been 
given in a general way, in the foregoing pages, to the inercas- 
ing complexity of the brain, especially to the relative size 
and markings of the cerebral hemispheres and cerebellum, 
as we rise from the fish to man. 
Organs of Sense.—While all animals, perhaps without 
exception, unless it be the root-barnacles, and a few other 
parasitic forms, have the sense of touch, which, in the lower 
Protozoa is so slight as to be compared with the contractility 
common to all living protoplasmic matter, whether existing 
in cellular tissue or one-celled, independent animals ; not all 
of the lower animals have, however, definite sense-organs. 
The Eye.—The most important of these are undoubtedly 
eyes, as they are the most commonly met with. The sim- 
plest form of eyes are perhaps those of the sea-anemone, in 
which there are, besides pigment cells forming a colored 
mass, refractive bodies which may break up the rays of light. 
impinging on the pigment spot, so that these creatures may 
be able to distinguish light from darkness. The next step 
in advance is where a pigment mass covers a series of refract- 
ive cells called ‘‘ crystalline rods ”’ or ‘‘ crystalline cones,”’ 
which are situated at the end of a nerve proceeding from 
the ‘‘ brain.’? Such simple eyes as these, often called ‘‘ eye- 
spots,’? may be observed in the flat worms, and they form 
the temporary eyes of many larval worms, Echinoderms 
and mollusks. In some nemertean worms, such as certain 
species of Polia and Nemertes, true eyes appear, but in the 
ringed worm, Neophanta celoz, Greef describes a remarka- 
