ANNELIDA. 



167 



tinguished, and form, as it were, but a single 

 organ ; lastly, there are cases in which only 

 one of the oars would seem to be developed. 

 If one were disposed to compare the loco- 

 motive system of the annelida with that of the 

 other articulate classes, the ventral oar should be 

 regarded as analogous to the members which in 

 the Crustacea, Insects, &c. are variously modified 

 to constitute the legs, the jaws, or the antenna? : 

 and the dorsal oar ought to be considered as 

 representing the appendages, which, though 

 wanting in the greater number of articulate 

 animals, yet acquire a considerable develop- 

 ment on the last two rings of the thoracic 

 segment of most insects and constitute the 

 wings. In this particular the annelida afford 

 an example of the greatest uniformity in the 

 development of the appendicular system in the 

 articulate division of the animal king-dom. 



Each oar is essentially composed of a fleshy 

 tubercle more or less prominent, which sup- 

 ports different productions of the integument, 

 incloses the bristles (c), and which is more 

 especially designated by the name of foot. 

 Towards the base of the setiferous tubercle 

 there is generally a membranous appendage, 

 sometimes filiform, sometimes lamelliform, 

 called the cirrus (d, e); lastly, it is also above 

 the margin and near the base of these organs 

 that the branchiae (f) take their origin, but in 

 general it is only the dorsal oar that supports 

 them. All the above parts may exist simul- 

 taneously, but it often happens that one or 

 more are atrophied to a greater or less degree, 

 or are altogether deficient; and this either 

 along the entire body or on certain segments 

 only. Thus in the terricolous annelida there 

 are no cirri ; in the hermellse they are pre- 

 sent on the ventral, but not on the dorsal oar ; 

 while in the cirrhatulae the reverse obtains. 



In most of the annelida errantia the setiferous 

 tubercle of both oars is wanting on the first 

 rings which follow the head, whilst the cirri 

 assume a very great development, and form the 

 appendages termed by systematic authors ten- 

 tacular cirri. ( Fig. 62, d.) 



A similar modification may be frequently 

 remarked in the composition of the appen- 

 dicular system of the last ring of the body, and 

 thence results a certain number of filiform pro- 

 ductions called styles. Lastly, the antennae of 

 the annelida, which must not be confounded 

 with the antennae of insects and Crustacea, may 

 also be considered as representing the cirri of 

 the dorsal oar of those rings, the union of which 

 constitutes the head.* 



The annelida pass in general a somewhat 

 stationary life, and a great number among 

 them remain constantly buried in the earth or 



* For further details regarding the external struc- 

 ture of the annelida the reader may consult the 

 excellent work of M. Savigny, intitled " Systeme 

 des Aunelides," principally of those found on the 

 coasts of Egypt and Syria; the article ' Vers' of 

 the Dictionnaire des Sciences Naturelles, torn. Ivii. 

 by M. De Blainville ; and a more recent publica- 

 tion on the same subject inserted in the Annales 

 des Sciences Naturelles, torn, xxviii, xxix, and xxx, 

 and in tbe second volume of the ' Rcchcrches pour 

 servir a 1'Hist. Nat. du littoral de la France, par 

 MM. Audouin et Milne Edwards.' 



enclosed in tubes formed by the mucus which 

 is secreted by the skin, and which, while hard- 

 ening, commonly agglutinates together frag- 

 ments of shells and sand. The formation of 

 these sheaths is very quick. I have seen them 

 fabricated in the course of a few hours. Some- 

 times they are of extreme tenuity, occasionally 

 they are as tough as thick leather, and there 

 are some which possess very considerable 

 hardness and are composed in great proportion 

 of carbonate of lime, like the shells of mol- 

 lusca. In the greater part of these animals 

 locomotion is produced by general undulations 

 of the body determined by contractions of a 

 layer of muscular fibres extending from one 

 ring to another, and fixed to the inner surface 

 of the skin. But in other species the change 

 of place is effected by the action of the feet, 

 of which we have spoken ; or by the contrac- 

 tion of the tentaculae which surround the 

 mouth, as in the terebellae, and which, by 

 shortening themselves, drag on the body of 

 the animal in the same manner as the arms of 

 the cephalopods : lastly, by the action of the 

 suckers with which the extremities of the body 

 are furnished. 



The bristles (fg. 63 and 64, r,) with which 

 thefeetof the annelida are provided, do not serve 

 merely as little levers to facilitate their move- 

 ments, but are also offensive arms, and their 

 structure is very curious. They differ con- 

 siderably from the hairs of other articulate 

 animals, which are nothing more than small 

 tubular prolongations of the epidermic layer. 

 By their mode of connexion with the integu- 

 ments and their mode of formation they ap- 

 pear to approach the hair of mammalia, but 

 their disposition is of a more complicated na- 

 ture. They are inclosed in sheaths provided 

 with muscular fibres, by the aid of which the 

 animal can protrude and retract them again : 

 in general, also, they are not merely simple 

 conical filaments, but their extremity is often 

 shaped like a harpoon, a lance, or a barbed 

 arrow, and the annelidan uses it to inflict a 

 wound upon its enemies.* 



Sensation. Tactile sensibility is considerable 

 in these animals, and it seems to reside prin- 

 cipally in the antennae, the cirri, and the 

 tentacula. They do not appear to possess a 

 sense of hearing, and there are many among 

 them which do not manifest any sign of sen- 

 sibility to light ; but in others, eyes (Jig. 62, a,) 

 exist, the number of which is sometimes very 

 considerable, but the structure very simple. 

 They are coloured points, (generally black,) and 

 situated on the dorsal aspect of the head or on 

 the cephalic sticker. In the setiferous anne- 

 lida there are never more than two pairs, but 

 in the hirudinidae or leeches their number 

 often increases to eight or ten. The anatomy 

 of these eyes has recently been studied by 

 Professor Muller of Berlin, and according to 

 his researches it would seem that these organs 

 do not contain a crystalline lens, or a trans- 

 parent body analogous to the vitreous cones of 



* See Observations sur Ics Foils des Annelides 

 ronsideres romme nioyon de Defense, par MM. 

 Audouin et Milne Edwards, op. cit. toiu. ii. p. 31. 



