<^81. 



THE ECHINODERMATA. 



83 



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 Asteroi'dea, and Echinoidea, no organs of visioii 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,*^' 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.'^' 



7 According to Krohn {MuUer''s Arch. 1839, 

 p. 348), the oesophageal ring of Sipunculus nudus 

 has two super-oesophageal gangUa blended to- 

 gether. These had aheady been observed by 

 Delle Chiaje (Memor. loc. cit. I. p. 15, Tav. I. 

 fig. 6. i.); but more lately Grube had taken them for 

 cartilaginous rudiments of the osseous circle 

 (MuUer^s Arch. 1837, p. 24-1). He has also con- 

 founded with the muscular system the two lateral 

 nerves of this ring, and its abdominal branch 

 which in its com'se 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 Forbex and Goodsir {Froriep\s 

 neue Not. No. 392, 1841, p. 279), the nervous sys- 

 tem of EchiuTus is composed of an oesophageal 

 ring, with an abdominal cord, from which pass off 

 asymmetrical branches. 



According to Quatrefag-es, 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 {Milller Zool. Dan. 

 Tab. CXXXI.) had ah-eady observed in Pteraster 

 militaris, were first regarded as eyes by Eliren 

 berg {Miiller^s Arch. 1834, p. 577, and Abhand. 

 d. Berl. Akad. 1835, p. 209, Taf. VIU. 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 m 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 Valentin, 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 ; 



brance : " It is a noticeable fact that the nervous 

 trunks of these animals tlnroughout 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 from alcohohc 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 rvfo-fiin- 

 briatus. It consists of two cerebral gangUa 



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 shght swellings along its course, which 

 may be regarded as ganglia ; they send nerves to 

 the uitegiuuents. 



This anatomist has also observed here a very 

 distinct splanchnic system of nervea. — Ed. 



