-5^^ 123, 124. 



THE TURBELLARIA. 



135 



CHAPTERS III. AND IV 



NERVOUS SYSTEM AND ORGANS OF SENSE. 



§ 123. 



The nervous system with the Turbellaria, is quite indistinct, for it has 

 not yet been observed in the small species, and in the larger ones its dis- 

 position is yet doubtful. A double ganglion in the cervical region appears 

 to form its central part, and from this nerves pass off in different direc- 

 tions.^^' 



§ 124. 



Among the organs of sense, those of vision are the most developed with 

 very many species. 



The red, brown, or black spots on the anterior extremity, two or 

 more in number, are not always simple pigment cells, ^^' but may be regard- 

 ed as eyes, for they have a cornea, — a light-refracting body surrounded 

 with pigment, and a nerve-bulb.'-^' 



As to the sense of touch, no special tactile organs have yet been found, 

 but the whole surface reacts sensitively from the lightest contact; and this 

 sensibility appears particularly prominent at the anterior extremity, which, 

 with many Dendrocoeli, is furnished with lobular and other appendages.*^' 



1 Ehrenberg has seen two disconnected ganglia 

 with Planaria lactea (Abhand. d. Berl. Akad. 

 1835, p. 243). With other Dendrocogli, as with 

 Planocera sargassicola, and pellucida, these two 

 ganglia are blended into one ; at least, the organ 

 which Mertens has here described as a Iieart, has 

 exactly the appearance of two united gansrlia (loc. 

 cit. Taf. I. fig. 6, Taf. II. fig. 3, m. or Isis 1830, Taf. 

 IX. fig. 3, c. m.). The liglit pulsations which this 

 author affirms to have here observed, are perhaps, 

 as Ehrenberg has supposed (loc. cit. p. 244), due to 

 the contractions of neighboring organs. According 

 to Schulze (loc. cit. p. 39), with Planaria torva, 

 the double central ganglion gives off two nerves, 

 which pass backwards on both sides of the intes- 

 tine. 



This double ganglion, situated in the cervical re- 

 gion, and the nervous filaments which it gives off, 

 have been demonstrated by Quatrefages (loc. cit. 

 p. 172, PI. IV.-VI.). 



1 Most commonly there are two eye-dots. With 

 Planocera, and Leptoplana, there arc many which 

 are grouped together, and with Polycelis nigra, 

 the whole anterior part of the body is covered with 

 them. In many small species, they appear to be 

 of a simple pigmentary nature. 



2 With Planaria laixtea, there is, between the 

 cornea-like bulging of the skin, and a semilunar, 

 pigment layer, a small, conical, transparent body, 

 corresponding exactly to a crystalline lens ; see 

 Ehrenberg, loc. cit. p. 243, and Schulze, loc. cit. 



* [ § 124, note 2.] See also Leidy (Proc. Acad. 

 Nat. Sc. Philad. HI. 1848, p. 248) on the eye-si)ecks 

 of Phagocata gracilis, a sub-genus made by him 

 from Planaria, and Schmidt (Die RhabdocoClen 

 Strudelwurmer,&c., p. 7, and Neue Beitrllge Zur 

 Naturgeach, der Wiirmer, &c., p. 11). Both of 

 these observers agree in considering these parts in 



p. 37. With Monocelis, these organs are very 

 remarkable, being composed of two eyes blended 

 uito one, and the simple and spherical ball of the 

 eye is filled, according to Orsted, with a transpa- 

 rent vitreous body, in which two conical crystalline 

 lenses are buried with their apices pointing in- 

 wards (loc. cit. p. 6, 56, Taf. I. fig. 1, 2, and in 

 the text, fig. 10). Orsted has distinctly seen two 

 optic nerves passing laterally to this organ. It is 

 quite remarkable that with one of the three known 

 species, the Monocelis unipunctata, the eye is 

 entirely without pigment. Ehrenberg affirms that 

 he has observed with Polycelis, many slar-like 

 ganglia in the middle of the anterior part of the 

 body, which are for the long row of eye-dots (loc. 

 cit. p. 243). 



For the eyes of the marine Ptenanoe, see also 

 Quatrefages, loc. cit. p. 178, PI. III. The organ 

 which with Monocoelis has been taken for an eye 

 by Orsted, appears to be, according to the re- 

 Searches of Frey and Leuckart (Beitr. p. 83, Taf. 

 I. fig. 18), an auditory organ. Tliat which Orsted 

 regarded a vitreous body, is an otolite, and his two 

 crystalline lenses, are two semicircular prolonga- 

 tions attached loop-like to the otolite. Frey and 

 Leuckart are also convinced that Convoluta par- 

 adoxa Orst., has a single auditive cajjsule, situated 

 on the median line of the cervical region, and con- 

 taining an otolite which floats in a lilac-colored 

 fluid ; see Beitr. loc. cit. p. 82, Taf. I. fig. 17.* 



3 There are contractile and anteuniform append- 



question as visual organs. Schmidt has often 

 failed to find anything like an otolite ; but, on the 

 other hand, has often found with various Derosto- 

 mum a complete visual apparatus. This point, 

 therefore, is still unsettled, unless, as Schmidt in- 

 geniously suggests, it may be that one organ serves 

 the functions of two separate senses. — Ed. 



