NEKVOUS SYSTEM OF THE ANNELIDANS. 



277 



and forms a prominent ridge underneath the head, that constitutes a 

 kind of upper lip. In the course of two or three days more, the anterior 

 cephalic lobe (136, 5, a) becomes perfectly distinct from the oculiferous 

 segment, and is much elongated, taking a cylindrical form, and consti- 

 tuting a very flexible median appendage, having all the characters of an 

 antenniform organ. Its axis is occupied by a canal that communicates 

 with the general cavity of the body ; and a fluid may be seen to circu- 

 late in its interior. The natatory cilia have almost entirely disap- 

 peared both from the neck and from the posterior extremity of the 

 body ; and the young Terebella in this condition presents itself exhibit- 

 ing all the characters of an Annelid belonging to the erratic group- 

 not, as yet, at all resembling any of the tubicolous genera, of which it 

 is a member. 



(711.) Having become deprived of the locomotive cilia with which 

 they were previously furnished, the larvae now cease swimming and 

 begin to enclose themselves in a kind of mucous substance, which gra- 

 dually solidifies, so as to form a cylindrical tube open at both extremi- 

 ties. The first period of their existence, during which they lead an 

 erratic life, then closes, and they begin to assume the habits of their 

 parents. The ventral oars, with their armature of terminal booklets, 

 are successively developed in a regular series from before backwards, as 

 additional segments are added to 

 the length of the body. The ten- 

 tacular appendages next begin to 

 be developed from the sides of the 

 head. But it is not before the body 

 has acquired thirty-eight or forty 

 pairs of feet that the branchial 

 apparatus makes its appearance, 

 under the form of two simple tu- 

 bercles developed from the lateral 

 regions of the neck ; these, how- 

 ever, rapidly enlarge, and soon 

 assume the functions which, in the 

 adult animal, they are destined to 

 perform. 



(712.) The structure of the 

 nervous system in the Annelida 

 conforms, in its arrangement, with 

 the general type common to the 

 articulated classes. A considerable 

 supra-cesophageal mass (fig. 137, 

 a a) represents the encephalon, in front of which are situated minute 

 ganglia (6, c, d, e), from whence nerves are derived to supply the prin- 

 cipal instruments of sensation connected with the cephalic portion of 



Plan of the nervous system in the 

 Dorsibranchiate Annelidans. (After 

 Quatrefages.) 



