§ 148. THE ANNELIDES. 159 
which are nearly blended together into one, and whose closely-successive 
ganglia correspond numerically with the segments of the body.” 
4. With those Chaetopodes which have external branchiae, the nervous 
system is most highly developed, but has wide variations as to its whole or its 
details, according to the more or less complicated structure of the cephalic 
extremity and segments of the body. With those species which are without 
antennae and eyes, the ventral medulla is composed of two contiguous 
cords the enlargements of which are indistinct and not sharply defined. 
These two cords are separated at the cephalic extremity, and terminate 
either, by a ganglion on each side without apparently forming by a commis- 
sure an oesophageal ring,” or by encompassing the cesophagus, and forming 
a ring through a ganglion lying upon it.© 
With some, the two parallel cords are without ganglia but are retinited 
at each segment of the body by two transverse threads. With others, 
this connection occurs through transverse threads and ganglia. There 
are many Branchiati with which the two cords are so closely contiguous 
that they are separated only by a longitudinal furrow. Their round or 
elongated ganglia are then common, and succeed each other at longer or 
shorter intervals.*? With an entire series of the Dorsibranchiati, the 
ventral ganglia are so closely approximated that the interganglionic cords 
appear wholly wanting.“ 
The brain is composed of only two ganglia, which are more or less blended 
into one with the Capitibranchiati, and with those Dorsibranchiati whose 
head is very slightly developed; “ while with the other Dorsibranchiati 
whose head is distinct and the eyes and tentacles very much developed, it 
is the product of the fusion of many ganglia.“® 
5 See Gruithuisen, in the Nov. Act. Acad. 
XIV. 1828, p. 412, Tab. XXV. fig. 3-5 (Chaeto- 
gaster diaphanus); Henle, in Miuller’s Arch. 
1837, p. 85, Taf. VI. fig. 2, 3, 8, x, y (Enchi- 
traeus); Roth, De Animalium invertebratorum 
systemate nervoso. Wirceburg, 1825, fig. 3 ; and 
Morren, loc. cit. p. 117, Tab. XIX.-XXIII. 
(Lumbricus terrestris). 
In the common Lumbricus, two pairs of nerves 
«(Nervi annulares) pass off laterally from the cen- 
tre of the ganglionic enlargements ; and between 
every two ganglia, exceptionally, there passes off 
another pair (Nervi interannulares) which are 
distributed to the transverse muscular septa ; see 
Morren loc. cit. The nervous system of Sternas- 
pis thalassemoides is quite different, and appears 
retrograded to the type of that of the Sipunculidae, 
for the ventral medulla consists only of a simple 
cord which is enlarged at the caudal extremity ; 
see Will, in Miller’s Arch. 1842, p. 427. 
6 Arenicola, Ammotrypane, and Terebella. 
7 Arenicola; see Grube, Zur. Anat. d. Kiemen- 
wiirmer, p. 17, Tab. I. fig. 7; and Stannius, in 
Mujller’s Arch. 1840, p. 379, Taf. XI. fig. 15. 
8 Ammotrypane ; see Rathkd, in the Nov. Act. 
Acad. XX. p. 197, Tab. X. fig. 14, 19. 
9 Sabella; see Wagner, Isis, 1832, p. 657, Taf. 
X. fig. 14; and Grube, Zur. Anat. d. Kiemenwur- 
mer, p. 30, Taf. II. fig. 16. 
10 Phyliodoce. Here, the transverse threads 
commence only at the border of the 7'h or 9th gan- 
glion. They alternate regularly with these and 
disappear towards the last segments of the body ; 
see Quatrefages, Ann. d. Sc, Nat. LL. 1844, p. 95, 
Pl. IL. fig. 2, 3. aig # 
ll Siphonostomum,Mimphitrite, Amphinone, 
Aricinella, Polynoe, 2} td Aphrodite. With Si- 
*[§ 148, note 13.) 
1849, p. 300 (Chloraema), — Ep. 
phonostomum, the ventral ganglia are very long ; 
see Rathké, Danzig. Schrift. loc. cit. p. 90, Taf. VI. 
fig. 3. Here, the peripheric nerves are given off 
from the interganglionic cords and not from the 
ganglia themselves. 
With Amphitrite, the ventral ganglia are long 
also, but from the fifth segment of the body they 
alternate with others that are round, so that each 
segment has two ganglia. Both of these ganglia 
furnish exclusively the peripheric nerves, but in 
front where the round ganglia are wanting, they 
are furnished also by the interganglionic cords ; 
see Rathke, loc. cit. p. 75, Taf. V. fig. 7, 15. With 
Aricinella (Quatrefages, loc. cit. p. 96, Pl. II. 
fig. 5), and Amphinome (Treviranus, Beobacht. 
ausd. Zoot. u. Physiol. 1839, p. 83, Taf. XI. fig. 
72), the ganglia are very closely set together. With 
Aphrodite, and Polynsé, the number of ventral 
ganglia exceeds that of the segments of the body ; 
see Grube, loc. cit. p. 66. 
12 Nereis, Eunice, Glycera ; see Wagner, in 
Isis, 1834, p. 133, Taf. I. fig. 11; Muller, in the 
Ann. d. 8c. Nat. XXII. 1981, p. 22, PL. IV. fig. 10 ; 
Rathké, De Bopyro et Nereide p. 41, Tab. IL. fig. 
18; Grube, Zur. Anat. d. Kiemenwirmer, p. 43, 
Taf. I. fig. 9; and Quatrefages, Ann. d. 8c. 
Nat. loc. cit. Pl. I. fig. 1, 2, 3. 
138 Amphitrite, Siphonostomum (Rathké, Dan- 
zig. Schrift. loc. cit. Taf. V. fig. 7,14, Taf. VI. fig. 
3), and Glycera (Quatrefages, Aun. d. Sc. Nat. 
loc. cit. p. 96, Pl. I. fig. 3).* 
14 Nereis, Eunice, and Phyllodoce ; see Mil- 
ler, Ann. d. Sc. Nat. loc. cit. Pl. IV. fig. 10; 
Rathké, De Bopyro et Nereide, p. 43, Tab. IT. fig. 
4, 5,135 and Quatrefages, loc. cit. p. 81, Pl. IL. 
fig. 1, 2, Pl. I. fig. 1. 
See Quatrefages, Ann. d. Sc. Nat. X. 1848, p. 47 (Hermelia), and XII. 
