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.

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    With the Sipunculidae, as with the other worm-like Echinoderms which approach the Annelids, the arrangement of the nervous system is quite different. . Here, the nervous ring is a simple, aganglionic thread extend- ing to the posterior end of the body, and may be regarded as the first trace of a ventral cord.® CHAPTER IV. ORGANS OF SENSE. §81. The sense of touch is well developed with the Echinoderms, and seems to have its seat in the oral tentacles, the ambulacra, and pedicellariae. With the Asteroidea, and Echinoidea, no organs of vision have yet been found. As such, however, have been regarded the red pigment dots sit- uated, with the former, at the extremity of their rays,(1) and with the latter, in the middle of the dorsal region upon five ocellary plates which alternate regularly with those of the genital organs.® These ocellary plates are perforated each by a very fine canal, through which passes a delicate fila- ment from-the main nerve for the pigment dot.® Although these pigment dots have thus a nervous connection, no proper organ to refract the light has yet been found in them.(4) 1 According to Krohn (Muller's Arch. 1839, p. 348), the oesophageal ring of Sipunculus nudus has two super-oesophageal ganglia blended to- gether. These had already been observed by Delle Chiaje (Memor. loc. cit. I. p. 15, Tav. I. fig. 6. i.)j but more lately Grube had taken them for cartilaginous rudiments of the osseous circle (Muller's Arch. 1837, p. 244). He has also con- founded with the muscular system the two lateral nerves of this ring, and its abdominal branch which in its course sends off laterally branches to the muscular layer and to the skin, and termi- nating at the end of the body in a swelling. Then, on the other hand, the filaments surrounding the digestive tube, and taken by him for nerves, appear to be only cellular fibres (loc. cit. p. 244, Taf. XI. fig. 4). According to Forbes and Goodsir (Froriep's neue Not. No. 392, 1841, p. 279), the nervous sys- tem of Echiurus is composed of an oesophageal ring, with an abdominal cord, from which pass off asymmetrical branches. brance : “ It is a noticeable fact that the nervous trunks of these animals throughout are contained in a sheath, which, after the maceration of its contents, has exactly the aspect of a blood-vessel.” The nervous system of these animals cannot, there- fore, be properly studied fi;om alcoholic speci- mens. — Ed. * [ § 80, note 7.] See also Blanchard (Ann. d. Sc. Nat. 1849, XII. p. 57), who has well made out the nervous system with Sipunculus rufo-Jim- briatus. It consists of two cerebral ganglia According to Quatrefages, Echiurus Gaertneri has an abdominal cord which possesses ganglia, and by this character the Echinidae approach the Annelida *, see Ann. d. Sc. Nat. VII. 1847, p. 332, PI. VI. fig. 4* 1 In the Clypeastridae and Echinidae. 2 These dots, which Vahl (Muller Zool. Dan. Tab. CXXXI.) had already observed in Ptcraster militarise were first regarded as eyes by Ehren berg (Muller's Arch. 1834, p. 577, and Abhand. d. Berl. Akad. 1835, p. 209, Taf. VIII. fig. 11, 12). He has seen in Asteracanthion violaceus, a small swelling at the extremity of the nerve of the ocel- lary dot. Forbes (Hist, of the Brit. Star-fishes, 1841, p. 152) first noticed these dots in Echinus, and their presence has been confirmed by Agassiz and Valentin (Monogr. loc. cit. p. 10, 100, PI. II. fig. 12, PI. IX. fig. 188, 189). 3 Valentine loc. cit. PI. IX. fig. 190. 4 Valentin has failed to discover in these organs a crystalline lens. Although in Echinus they are upon the back, and therefore favorable to vision ; united so as to form a single cordiform mass — the brain, which is situated under the muscles of the proboscis. From this brain passes off a cord on each side, forming a collar about the oesophagus } these unite below, and then continue as a ventral cord to the posterior extremity of the body. This cord has slight swellings along its course, which may be regarded as ganglia *, they send nerves to the integuments. This anatomist has also observed here a very distinct splanchnic system of nerves. — Ed.