168 



ANNELIDA. 



the Crustacea and insecta, but consist simply of 

 a terminal ganglion of the optic nerve covered 

 by a layer of black pigment and placed imme- 

 diately beneath the integument, which is thin and 

 transparent at that part.* 



Nervous system. In like manner the ner- 

 vous system of the annelida 

 Fig. 65. is verv simple. It occupies 



._ a the middle line of the ventral 



c aspect of the body, and con- 

 sists of a double series of mi- 

 nute ganglions of medullary 

 matter, more or less inti- 

 mately united or even 

 blended together, and equal 

 in number to the number 

 of rings. ( See Jig. 65. repre- 

 senting the nervous system 

 of the aphrodita aculeata). 

 The ganglions give origin 

 to lateral branches, and 

 are connected together by 

 two chords of communi- 

 cation, sometimes separate, 

 sometimes united into a sin- 

 gle trunk, so as to constitute a 

 longitudinal chain extended 

 through the entire length of 

 the body. The first of these 

 ganglions (a) is lodged in the 

 head, or at least at the ante- 

 rior extremity of the animal, 

 in front of or above the di- 

 gestive tube; the rest are 

 placed below that canal ; 

 whence it results that the two 

 nervous chords which form 

 the media of communi- 

 cation between the cephalic 

 ganglion and the first of the 

 sub-cesophageal series pass 

 along the sides of the oeso- 

 phagus, and form around that 

 canal a species of collar or 

 ring ; a character which is common to all the 

 articulate animals. f 



Organs of' digestion. The alimentary canal 

 in the annelida extends from one end of the 

 body to the other, and has an external com- 

 munication at both extremities. The mouth is 

 generally provided with a projectile proboscis, 

 which is formed by the anterior portion of the 

 digestive canal, which can be inverted and pro- 

 truded like the finger of a glove, and possesses 

 muscles for the express object of effecting these 

 movements (see Jig. 66, which represents the 



Fig. 66. 



* See Annales des Sciences Nat. torn. xxii. 



+ See Cuvier, Anat. Comparee, torn. i. ; Trevi- 

 ranus, iiber der stachlichten Aphrodite, Zeitschrift 

 f iir Physiologic, 3 Band ; Moquin Tandon, *' Mo- 

 nograph, des Hirudines/' Morrem, " Sur le Lom- 

 bric/' &c. 



proboscis of a phyllodoce, and^g. 67, that of a 

 nereis). The surface is frequently beset with 

 small papillae, and its extremity armed with 



Fig. 67. 



horny jaws (ni), the disposition of which varies 

 in different genera. It is to be observed that 

 these jaws are almost always placed laterally 

 like the mandibles of other articulate animals, 

 and cannot act upon one another in the direc- 

 tion of the axis of the body, as in the vertebrata, 

 but are not to be regarded as analogous to the 

 mandibles and maxillae of insects and Crustacea. 

 In their structure, the jaws of the annelida ap- 

 proximate rather to the solid plates with which 

 the interior of the stomach in some Crustacea is 

 provided, and to the hooks which arm the 

 mouth of certain gasteropodous molluscs. This 

 conformation of the oral apparatus is met 

 with only in the annelida errantia ; in the 

 annelida terricola there is scarcely a vestige of 

 a proboscis, and never any teeth or jaws. In 

 the annelida suctoria, the mouth, which is 

 placed at the bottom of the cephalic sucker, is 

 also occasionally protruded in the form of a 

 small tubular proboscis, and in other species 

 its margins are armed with little horny jaws ; 

 lastly, in the annelida tubicola, nothing of the 

 kind is to be seen, but in general the superior 

 border of the mouth forms a sort of projecting 

 lip, which is provided with long tentacles, 

 sometimes simple and filiform, sometimes pec- 

 tinated and resembling tufts. In certain erratic 

 annelida, the Agliope, for example, there are 

 also found around the mouth small tentacula, 

 which are quite distinct from the tentacular 

 cirri, and which appear to be analogpus to the 

 appendages of which we have just made 

 mention. 



The oesophagus which succeeds the pro- 

 boscis or mouth presents nothing worthy of 

 notice, but it is in general quite distinct from 

 the stomach. The conformation of the latter 

 organ varies much. Sometimes the stomach is 

 a simple enteroid tube (as in the nereida and 

 terebellae) ; sometimes it is composed of two 

 pouches, of which the first is membranous 

 and may be compared to a crop, while the 

 second is muscular and is analogous to a 

 gizzard, as, for example, in the lumbrici, 

 thalassenuB. In other cases the stomach pre- 

 sents on either side a succession of enlarge- 

 ments which have in general the form of 

 rounded cells, but which sometimes consti- 

 tute sacs or vast and much elongated ccecums, 

 (as in some hirudihes,/gs. 68 and 69.) Lastly, 



