$ 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 Asteroidea, and Hchinoidea, 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,® 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 (Miiller’s Arch. 1839, 
p. 348), the oesophageal ring of Sipunculus nudus 
ee two super. yh 
gether. These had already been observed by 
Delle Chiaje (Memor. loc. cit. I. p. 15, Tav. I. 
fig. 6. i.); but more lately Grube had taken them for 
eartilaginous rudiments of the osseous circle 
{Miller'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 cesophageal 
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 from alcoholic speci- 
mens. — Ep. 
* [§ 80, note 7.] See also Blanchard (Ann. d. 
Sc. Nat. 1849, XII. p. 57), who has well made out 
the nervous system with Sipunculus rufozfim- 
driatus. 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, Pl. V1. fig. 4.* 
1 In the Clypeastridae and Echinidae. 
2 These dots, which Vahl (Muller Zool. Dan. 
Tab. CXXXI.) had already gbserved in Pteraster 
militaris, were first regarded as eyes by Ehren 
berg (Miller’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 vonfirmed by Agassiz 
and Valentin (Monogr. luc. cit. p. 10, 100, Pl. 
Il. fig. 12, Pl. IX. fig. 188, 189). 
8 Valentin, loc. cit. Pl. IX. tig. 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 cesophagus ; 
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. 
