. Outlines of zoology. Zoology. STRUCTURE OF ARENICOLA. 203 the next nineteen have rudiments. The dorsal part con- sists of a tuft of bristles, whose bases are enclosed in a sac;—the ventral part, separated by a short interval, bears several hooks. Nervous System. The nervous system is in its general features like that of the earthworm, but ganglia are not developed. In the ventral nerve cord, the ring round the gullet, and the slight cerebral enlarge- ment which represents a brain, nerve cells occur diffusely scattered among the nerve fibres. Along the dorsal sur- face of the nerve cord run t
. Outlines of zoology. Zoology. STRUCTURE OF ARENICOLA. 203 the next nineteen have rudiments. The dorsal part con- sists of a tuft of bristles, whose bases are enclosed in a sac;—the ventral part, separated by a short interval, bears several hooks. Nervous System. The nervous system is in its general features like that of the earthworm, but ganglia are not developed. In the ventral nerve cord, the ring round the gullet, and the slight cerebral enlarge- ment which represents a brain, nerve cells occur diffusely scattered among the nerve fibres. Along the dorsal sur- face of the nerve cord run two " giant fibres " like those in the Fig. 65.—Anterior part of nervous system in Arenicola. (After VoGT and Yung.) c., Cerebral part on dorsal surface ; (, oesophageal ring; g., gullet : , ventral nerve cord ; /.«., lateral nerves ; d/., otocyst. particles ; in A. Grubii tain intrinsic otoliths of In some species at least, the head lobe is distinctly sensory and there are two ciliated "neck ; Otherwise sense organs are represented only by a pair of otocyst sacs, one on each side of the oesophageal nerve ring. These sacs, like those which occur in many otlier Invertebrates, seem to have to do rather with the direction of the animal's movements than with hearing. Prof. Ehlers notes an in- teresting series :—In A. Claparedii, there are simply two open grooves ; in A. inarina, the sacs have open necks and contain foreign and A. aiiti/lensis, the sacs are closed and con- lime. I^ond Canal. The mouth is at the end of a protrusible cup-like proboscis; the gullet has smooth walls, and bears an an- terior and a larger posterior pair of glands which secrete a yellowish fluid perhaps digestive; the succeeding part of the gut is covered with yellow cells and many blood vessels, and is divided into rings ; the terminal portion is full of sand from which the nutritive matter has been absorbed ; the anus is at the very end. T/ie Body
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
