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. 



BALANCING ORGANS 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, e.g., 

 the Common Lob -Worm (Arenicola pisca- 

 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. I042 .- Front Part of permanently retained in three kinds of Lob- 

 centrai Nervous system of Lob- Worm. In one of these (A. ClaparedU} there 



Worm (A remcolct ptscatorum], ... 



enlarged, g., Guiiet (in cross is simply 2i pair of depressions on the head, 



section) traversing nerve -ring; . \ t A , , \ 1*1 



ot. % otocysts in close connection in another (A. piscatorum) otocysts which are 



sti11 in communication with the exterior, and 

 in a third (A. GmbH) 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. 



BALANCING ORGANS IN MOLLUSCS (MOLLUSCA). 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 



