Class-book of physiology : for the use of schools and families : comprising the structure and functions of the organs of man, illustrated by comparative reference to those of inferior animals . elopmentof the nervous system in different groups of animals, andby noting the powers of its simplest, compared with itsmost perfect developments. 274. One of the simplest forms of thenervous system is found in the ascidia,{fig. 42,) one of the lowest of the class ofmollusca. In this and similar animals,we find only a single ganglion or nervouscentre. At a is seen the orifice by whichwater enters for su
Class-book of physiology : for the use of schools and families : comprising the structure and functions of the organs of man, illustrated by comparative reference to those of inferior animals . elopmentof the nervous system in different groups of animals, andby noting the powers of its simplest, compared with itsmost perfect developments. 274. One of the simplest forms of thenervous system is found in the ascidia,{fig. 42,) one of the lowest of the class ofmollusca. In this and similar animals,we find only a single ganglion or nervouscentre. At a is seen the orifice by whichwater enters for supplying the stomachwith food, and at b that through whichit passes out again. Between these ori-fices is the single ganglion, c, which sendsfilaments to both orifices, and also over Fthe surface of the envelop or mantle, d. TEM 0F As«dia. What are the centres of reflex action 1 What particular advantage maybe supposed to result from this arrangement? How may the true nature ofreflex action be more easily comprehended ? In what animal is one of thesimplest forms of the nervous system found 1 Describe the nervous system ofthe ascidia and the phenomena of life exhibited by that animal. 9*. 122 CLASS-BOOK OF PHYSIOLOGY. These animals are mostly fixed to one spot during theirwhole existence. The continual entrance and exit of thecurrents of water constitute the only phenomena of lifewhich they exhibit, except when the current draws in aninjurious substance. The mantle then contracts, causinga jet of water to issue from each orifice, and throw off theoffending material. This little animal has no eyes or otherorgan of special sense. The small tentacula or feelers, atthe upper orifice, are the only parts which seem to bepeculiarly sensitive; and the irritation caused by the con-tact of a hard substance with these, or with the generalsurface of the body, produces an instinctive contraction ofthe mantle, for the purpose of getting rid of the irritatingcause. This contraction can only be
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