PLANAKIJE. 151 



motions are exceedingly active, and it glides along the plane upon 

 which it moves with a rapid and equable pace of which the observer 

 would scarcely expect so simple a being to be capable, or, by means of 

 two terminal suckers, progresses in the manner of a leech. 



(399.) Many of the larger marine species are able to swim freely in 

 the sea by the aid of violent flappings of the broad margins of their 

 bodies, whereby they beat the water much in the same way as the broad 

 fins of a skate movements which it would be difficult to explain, except 

 by admitting the existence of a subcutaneous plane of muscular fibres, 

 such as is described by M. de Quatrefages as being recognizable in some 

 species. 



(400.) Although the existence of a nervous system in the Planariae 

 has been doubted by some observers, the researches of M. de Quatrefages 

 assure us of its presence in many species. It consists of two ganglions, 

 more or less intimately united, which are situated in the mesial line 

 near the anterior part of the body. This double ganglion, which may 

 be called the brain, and which is sometimes visible to the naked eye, is 

 lodged in a special lacuna or cavity, recognizable from its transparent 

 outline, and is seen to give off nervous filaments in various directions to 

 different parts of the body. 



(401.) M. Blan chard*, in dissecting a large individual belonging to 

 this group, not only found the two cerebroid nervous centres, abo^e 

 alluded to, closely approximated, but observed that they gave origin to 

 two long cords, which exhibited at regular intervals a series of minute 

 ganglia, thus clearly approximating to the type of structure that charac- 

 terizes the lower forms of the annulose worms. 



(402.) Many species of Planarise possess two red specks upon the 

 anterior part of the body, which, as in other cases, have been unhesi- 

 tatingly pronounced to be eyes, although their claim to such an appella- 

 tion is not only unsubstantiated by any proofs derivable from their 

 structure, but completely negatived by experiments, which go to prove 

 that, in the pursuit of prey, no power of detecting the proximity of their 

 food, by the exercise of sight, is possessed by any of them. 



(403.) The phenomena which have been observed, connected with the 

 multiplication of the Planarise by division, are analogous to those which 

 we have witnessed in other acrite animals ; for it has been proved that, 

 if an individual be cut to pieces, every portion continues to live and feel, 

 from whatever part of the .body it may be taken ; and, what is not a 

 little remarkable, each piece, even if it be the end of the tail, as soon 

 as the first moment of pain and irritation has passed, begins to move in 

 the same direction as that in which the entire animal was advancing, as 

 if the body was actuated throughout by the same impulse ; and, more- 

 over, every division, even if it is not more than the eighth or tenth part 

 of the creature, will become complete and perfect in all its organs. 

 * Sur 1'Organisation des Vers, Ann. des Sc. Nat. 1847. 



