§ 148. THE ANNELIDES. 159 



wWch 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 

 «ither, by a ganglion on each side without appai-ently 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 reunited 

 at each segment of the body by two transverse threads. '''* With others, 

 this connection occurs through transverse threads and ganglia.""' There 

 are many Bi'anchiati 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 Nor. Act. Acad. /JÄowoÄ^omMm, the ventral ganglia are very long: 

 XIV. 1828, p. 412, Tab. XXV. fig. 3-5 {Chaeto- see Äart^-e, Danzig. Schrift. 'loc. cit. p. 90, Taf. VI. 

 gaster diaphanus) ; Henle, in Mailer's Arch. fig. 3. Here, the peripheric nerves are given off 

 1837, p. 85, Taf. VI. fig. 2, 3, 8, x, y (Enchi- from the interganglionic cords and not from the 

 traeus) ; Roth, De Animalium invertebratorum ganglia themselves. 



systemate nervoso. Wirceburg, 1825, fig. 3 ; and With Amphitrite, the ventral ganglia are long 



Morren, loc. cit. p. 117, Tab. XIX.-XXIII. also, but from the fifth segment of the body they 



iLumbricus terrestris). alternate with others that are round, so that each 



In the common Lu?/)brjcas, two pairs of nerves segment has two ganglia. Both of these ganglia 



<7V"ert)j arenit/are«) pass off laterally from the cen- fiu-uish exclusively the peripheric nerves, but in 



tre of the ganglionic enlargements ; and between front where Wie round ganglia are wanting, they 



every two ganglia, exceptionally, there passes off are furnished also by the interganglionic cords ; 



another pair {Nervi interannulares) which are see Rathke, loc. cit. p. 75, Taf. V. fig. 7, 15. With 



distributed to the transverse muscular septa ; see Aricinella {Quatrefaifes, loc. cit. p. 96, PI. II. 



JVfo/Tew loc. cit. The nervous system of 5;ernas- fig. 5), and Amphinome (Treviranus, Beobacht. 



j;j« i/ta/assemoiies is quite different, and appears aus d. Zoot. u. Physiol. 1839, p. 83, Taf. XI. fi". 



retrograded to the type of that of the Sipuuculidae, 72), the ganglia are Very closely set together. With 



fur the ventral medulla consists only of a simple Aphrodite, and Poli/noe, the nmnber of ventral 



cord which is enlarged at the caudal extremity ; ganglia exceeds that of the segments of the body : 



see fVill, in Muller's Arch. 1842, p. 427. see Grube, loc. cit. p. 66. 



6 Arenicola, Ammotrypane, and Terebella. 12 Nereis, Eunice, Glycera ; see Was^ner, in 



7 Arenicola; see Grube, Zur. Anat. d. Kidmen- Isis, 1834, p. 133, Taf. I. fig. 11 ; Müller, in the 

 ■Würmer, p. 17, Tab. I. fig. 7 ; and Stannius, in Ann. d. Sc. Nat. XXII. 1831, p. 22, PI. IV. fig. 10 ; 

 Muller's Arch. 1840, p. 379, Taf. XI. fig. 15. Rathke, De Bopyro et Nereide p. 41, Tab. II. fig. 



8 Ammotrypane ; see Rathkd, in the Nov. Act. 13 ; Grube, Zui". Anat. d. Kiemenwürmer, p. 43, 

 Acad. XX. p. 197, Tab. X. fig. 14, 19. Taf. II. fig. 9 ; and quatrefages, Ann. d. Sc. 



9 Sabella ; see Wagner, Isis, 1832, p. 657, Taf. Nat. loc. cit. PI. I. fig. 1, 2, 3. 



X. fig. 14 ; and Grube, Zur. Anat. d. Kiemenwur- 13 Amphitrite, Siphonostomum {Rathki, Dan- 



luer, p. 30, Taf. II. fig. 16. zig. Schrift, loc. cit. Taf. V. fig. 7, 14, Taf. VI. fig. 



10 Phyllodoce. Here, the transverse threads 3), and Glycera (Quatrefages, Ann. d. Sc. Nat. 

 commence only at the border of the 7''' or 9th gan- loc. cit. p. 96, PI. I. fig. 3j.* ~ 



glion. They alternate regularly with these and 14 Nereis, Eunice, &ud Phyllodoce; seeMül- 



disappear towards the last segments of the body ; ler, Ann. d. Sc. Nat. loc. cit. PI. IV. fig. 10 ; 



see quatrefages, Ann. d. Sc. Nat. H. 1844, p. 95, Rathke, De Bopyi-o et Nereide, p. 43, Tab. II. fig. 



PI. II. fig. 2, 3. 4, 5, 13 ; and Quatrefages, loc. cit. p. 81, PI. I. 



11 Siphonostomum, Amphitrite, Amphinome, fig. 1, 2, PI. 11. fig. 1." 

 Aricinella, Polynoe, and Aphrodite. With Si- 



* [§ 148, note 13.] See Quatrefages, Ann. d. Sc. Nat. X. 1848, p. 47 (He rmc I la),, anil XU. 

 1849, p. 300 (Chloraema). — ED. 



