Comparative anatomy / by C.Th. v. Siebold and H. Stannius ; translated from the German, and edited with notes and additions recording the recent progress of the science by Waldo I. Burnett.

  • Karl Theodor Ernst von Siebold
Date:
1854
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Licence: Public Domain Mark

Credit: Comparative anatomy / by C.Th. v. Siebold and H. Stannius ; translated from the German, and edited with notes and additions recording the recent progress of the science by Waldo I. Burnett. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

    139/482 (page 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.(1) § 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.(2) 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 Dendrocoeli, as with Planocerasargassicola,ai\d pellucida, these two ganglia are blended into one 5 at least, the organ which Mertens has here described as a heart, has exactly the appearance of two united ganglia (loc. cit. Taf. I. fig. 6, Taf. II. fig. 3, ra. or Isis 1836, Taf. IX. fig. 3, c. m.). The light 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 lactea, there is, between the cornea-like bulging of the skitf, and a semilunar, pigment layer, a small, conical, transparent body, corresponding exactly to a crystalline lens j see Ehrenberg, loc. cit. p. 243, and Schulze, loc. cit. * [ § 124, note 2.] See also Leidy (Proc. Acad. Nat. Sc. Philad. m. 1848, p. 248) on the eye-specks of Phagocata gracilis, a sub-genus made by him from Planaria, and Schmidt (Die Rhabdocoglen Strudelwiirmer,&c., p. 7, and Neue Beitrilge Zur Naturgesch, 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 into 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 star-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 Planaria*, 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. That 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- ado xa- Orst., has a single auditive capsule, situated on the median line of the cervical region, and con- taining an otolite which floats in a lilac-colored fluid 5 see Beitr. loc. cit. p. 82, Taf. I. fig. 17.* 3 There are contractile and antenniform append- question as visual organs. Schmidt has often failed to find anything like an otolite 5 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.