34 NERVOUS SYSTEM AND SENSE-ORGANS 
of the water, reflex modification of the swimming movements 
will take place, and the upward course will be altered into a 
downward one. 
BaLancinc OrcGaNns IN SEGMENTED Worms (ANNELIDA).— 
Members of this group commonly react very quickly to jolting 
or agitation of the surrounding medium, and this may lead to 
movements promoting escape from danger. Earth-Worms, for 
example, when partly protruding from their burrows, will often 
draw back with extreme rapidity on the approach of a heavy 
footstep. The skin is no doubt the sense-organ in this case, 
but we are ignorant as to details. A few Annelids, however, 
have a pair of otocysts in the front part of the body, as, ¢.g., 
the Common Lob-Worm (Avenzcola pusca- 
torum.), where they are closely connected with 
the brain (fig. 1042). They have undoubtedly 
been evolved from pits in the ectoderm like 
the similar sacs found in some of the jelly- 
fish, and three stages in this evolution are 
Fig. 1042 Front Part of Permanently retained in three kinds of Lob- 
Central Nervous Systemof Lob- Worm. In one of these (4. Claparedzz) there 
Worm (Arenicola piscatorum), 
enlarged. g, Gullet (in cross iS simply a pair of depressions on the head, 
Ticose dudes cacags-. 1G aROLneE (A. prscatorum) otocysts which are 
ae eae canals teextenor stil] in communication with the exterior, and 
in a third (4. Grudzz) closed otocysts. The 
otoliths of the second species are minute sand grains taken in 
from the exterior, while those of the third are calcareous par- 
ticles secreted by the ectoderm. 
BaLtancinG Orcans In Mo ttuscs (Mottusca).—Most Mol- 
luscs possess a pair of otocysts, developed as pits in the ectoderm, 
which become closed and travel inwards to the neighbourhood of 
the foot. They are attached to the foot-ganglia,; although their 
nerve-supply is derived from the brain (see fig. 1021, p. 17). 
It occasionally happens in Bivalves that the communication with 
the exterior is retained. The lining of these organs partly con- 
sists of sense-cells provided with stiff processes, and one or more 
calcareous otoliths are present. 
The otocysts of Cuttle-Fishes and their allies are lodged in 
the gristly case which surrounds the thickened nerve-ring, pretty 
much as in backboned animals the corresponding organs are 
sheltered in gristly or bony capsules that form part of the wall 
