640 



ZOOLOGY. 



tailed larval Ascidian presents features which apparently 

 anticipate the state of things existing among the lower ver- 

 tebrates, snch as the laucelet. 



In the last-named animal the nervous cord has a dorsal 

 position — i.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 increas- 

 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, iinless 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, indejiendent 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- 

 jilest 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 moUusks. In some nemertean worms, such as certain 

 species of Polia and Ncmertes, true eyes appear, but in the 

 ringed worm, Ncoplicmta celox, Greef describes a remarka- 



