<$> 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 Chactopodes 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 spet-ies 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 oesophagus, and forming 

 a ring through a ganglion lying upon it.**' 



With some, the two parallel cords are without ganglia but are reiinited 

 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.*''" 



•'See Gruithuisen, in the Nov. Act. Acad. /;Aonos<omf(m, the ventral ganglia are very long ; 



XIV. 1S2S, p. 412, Tab. XXV. fig. .3-5 (.Ckaeto- see RartAe, Danzig. Schrift.'loc. cit. p. 90, Taf. VI. 



gaster diaphamis) ; Henle, in Muller^s Arch. fig. 3. Here, the periplieric nerves are given off 



1S37, p. 85, Tiif. VI. fig. 2, .3, 8, x, y (Enchi- from tlie interganglionic cords and not from the 



traeus); Rotli, De Animalium iuvertebratorum ganglia themselves. 



systematB nervosj. Wirc-jburg, 1825, fig. 3 ; and Witli Amphitrite, the ventral ganglia are long 



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



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



In the commonLuznftrifius, two pairs of nerves segment has two ganglia. Both of these ganglia 



(iVeruj anKHteres) pass off laterally fr.jm the cen- furnish exclusively the peripheric nerves, but in 



tre of the ganglionic ei.largL'm.nits ; and between front where the round ganglia are wanting, they 



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



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



distributed to the traanverse muscular septa ; see Aricinella ((^uatrefases, loc. cit. p. 96, PI. II. 



Morren loc. cit. The nervous system of Sternas- fig. 5), and Ainphitiome {treviranus, Beobacht. 



pis tkalassemoides is quite Uillerent, and appears aus d. Zoot. u. Physiol. 18oJ, p. 83, Taf. XI. fig. 



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



fur the ventral medulla consists only of a simple Aphrodite, and Po/yiioi', llie number of ventral 



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



see ^fill, in Muller''s Arch. 1842, p. 427. see Grube, loc. cit. p. Go. 



6 Arenicola, Arnmotrypane, and Terebella. 1^ Nereis, Eunice, Glijcera ; see Was^ner, in 



I Arenicola; see Grube, Zur. Anat. d. Kiemen- Xsis, 1834, p. 133, Taf. I. fig. 11 ; Midler, in the 

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

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



8 Arnmotrypane ; see RatMi, in the Nov. Act. 13 ; Grube, Zui-. Anat. d. Kiemenwiirmer, p. 43, 

 Acad. XX. p. 197, Tab. X. fig. 14, 19. Taf. II. fig. 9 ; and Quatre/a:res, Ami. d. Sc. 



9 Sabetla ; see IVa^ner, Isis, 1832, p. 657, Taf. Nat. loc. cit. PI. I. fig. 1, 2, 3. 



X. fig. 14 ; and Grube, Zur. Anat. d. Kiemenwur- I'j Amphitrite, Siphonostomrrm (Rathk^, Ran- 



mer, p. 30, Taf. II. fig. 16. zig. Schrift. lie. cit. Taf. V. fig. 7, 14, Taf. VI. fig. 



10 PhyUodoce. Ilere, the transverse threads 3), and Glycera (^uaire/azes, Ann. d. Sc. Nai. 

 commence only at the border of the 7^1' or 9''' gan- loc. cit. p. 96, PI. I. fig. 3).* 



glion. They alternate regularly with these and 'ii Nereis, Eunice, axvX PhyUodoce ; se»Mul- 



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



see quatrefa^es, Ann. d. Sc. Nat. II. 1844, p. 95, Rathki, De Bopyro et Nereide, p. 43, Tab. II. fig. 



PI. li. fig. 2, 3. .1, 5, i:j ; and quatrefagcs, loc. cit. p. 81, PI. I. 



II Sipkonostomum, Amphitrite, Amphinome, fig. 1, 2, PI. II. fig. 1. 

 Aricinella, Polynoe, and Aphrodite. With Si- 



* [ § 148, note 13^ See Quatrefages, Ann. d. Sc. Nat. X. 1848, p. 47 (HcrmeUa), and XII. 

 1849, p. 300 {Chloraema). — Ed. 



