82 
system presents a cerebral ganglion variable in size, 
and a knotted spinal marrow, which has generally 
one ganglion in the corslet, two in the thorax, and 
four, six, or more, in the abdomen. In some species 
of neuroptera the optic nerves are larger than the 
brain itself °.” 
Amongst the mollusca the ganglia diminish in 
number, and gradually disappear. The nervous ring 
surrounding the cesophagus is either repeated, as in 
the leech, or imperfectly repeated, as in articulata. 
In holothuriz and siphunculi slightly serpentine 
threads are seen radiating from the nervous collar 
round the cesophagus, one passing between each of 
five pair of longitudinal muscles ¢. 
“ Cuvier was the first to remark that a whitish 
thread-like ring surrounds the commencement of 
the oesophagus in the asterias, siphunculi, and some 
holothuriz. We here look for the first definite ex- 
istence of the nervous system °. 
In the substance of zoophytes and of medusz the 
most powerful microscopes have discovered only a 
gelatinous mass composed of minute globules: nei- 
ther muscular nor nervous fibre being traceable. 
Trembley remarked in the fresh water polype an in- 
clination to the light. Cavolini observed in gor- 
goniz and sertulariz an evident aversion to it‘. 
The motions of the hedysarum gyrans when af- 
fected by the sun’s rays; of the Dionza muscipula 
when touched; of leaves when reversed, or placed 
© Pag. 53. 4 Pag. 50. e Pag. 44. tPaaeid3. 
