Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol 1: A-Dea
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit25759/179/
167 
ANNELIDA. 
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 antennæ : 
and the dorsal oar ought to lie 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 kingdom. 
Each oar is essentially composed of a fleshy 
tubercle more or less prominent, which sup¬ 
ports different productions of the integument, 
incloses the bristles (r), 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 hermellæ they are pre¬ 
sent on the ventral, but not on the dorsal oar; 
while in the cirrhatulæ 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 fihform pro¬ 
ductions called styles. Lastly, the antennæ of 
the annelida, which must not be confounded 
with the antennæ 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 “ Système 
des Annelides,” principally of those found on the 
coasts of Egypt and Syria ; the art:cle < Vers7 of 
the Dictionnaire des Sciences Naturelles, tom. Ivii. 
by M. De Blainville ; and a more recent publica¬ 
tion on the same subject inserted in the Annales 
des Sciences Naturelles, tom. xxviii, xxix, and xxx, 
and in tue second volume of the * Recherches pour 
servir à 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 tentaculæ which surround the 
mouth, as in the terebellæ, 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 (Jig. 63 and 64, c,) with which 
the feet of 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 antennæ, 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 sucker. In the setiferous anne¬ 
lida there are never more than two pairs, but 
in the hirudinidæ or leeches their number 
often increases to eight or ten. The anatomy 
of these eyes has recently been studied by 
Professor Müller 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 les Poils des Annelidrs 
considérés comme moyen de Defense, par MM. 
A udouin et Milne Edwards, op. cit. tom, ii, p. 31.
        

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