OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
275 
tended along the middle line of the dorsal surface of the body beneath the muscles 
of the segments, and immediately above the alimentary canal, from which it is sepa- 
rated, in the Chilognatha, only by a very delicate peritoneal membrane. It is attached 
on the upper surface to the median line of the segments, by means of delicate suspen- 
sory muscles, one pair in each segment, as in the larvae of insects. At its sides it has 
broad triangular muscles, which are collected into narrow fasciculi, and attached 
to the sides of the body, like the corresponding muscles, the aloe cordis already well 
known in insects. These muscles are formed of two sets of fibres attached to the 
sides and termination of each chamber. The anterior set is much the largest, and 
proceeds from the sides of the anterior half of each chamber, along which its fibres 
are extended, and intermingled with those of the external tunic of the organ. These 
fibres are gradually collected into fasciculi, on each side, which pass forwards, and are 
attached to the anterior lateral margin of the segment. These muscles assist to 
dilate the chamber, and to draw it forwards in the segment, while the ventricular, or 
contractile action is performed by the structure of the organ itself in the segment 
next behind it. The second set of muscles originates from the posterior lateral part of 
each chamber. This fasciculus of fibres is smaller than the first, and passes back- 
wards and outwards, and is also attached to the margin of the segment, and seems 
to be the first to act in dilating the chamber. These structures exist in the whole of 
the Myriapoda, Arachnida, and Insects, and mainly effect the auricular, or dilating 
action of the heart, which, in these classes, is not a simple passive act, or the result 
solely of a relaxation of its own structure. The contractile, or ventricular action of 
the organ, is performed entirely by its own fibres, the structure and arrangement of 
which I shall describe in the Chilopoda. 
The chambers of the heart are separated from each other by constrictions, or redu- 
plications of the muscular tunics, as shown by Straus in insects, but these constric- 
tions are only partial, and far less perfect than in insects, or in the Chilopoda, the 
most perfect Myriapods. Their rudimentary condition very much resembles that of 
the chambers of the heart in some of the low r est forms of the larvae of Dipterous and 
Hymenopterous insects, in which the heart is scarcely more than an elongated vessel, 
very slightly constricted in each segment, and almost as simple as the dorsal vessel 
of some of the Annelida. 
At each constriction of the heart in the Iulidae (PI. XIII. fig. 1 Q.f), between two 
chambers, there are two transverse lateral orifices, as in insects, through which the 
blood enters the organ. Whether these orifices in the Iulidae are the terminations of 
delicate veins, as I shall hereafter have occasion to show in the Arachnida, or whether 
they are simple apertures, that admit the blood from venous sinuses in the body, I 
am not certain. Most of my observations lead me to believe that they are the inlets 
of the venous trunks that bring back the blood to the heart. The vessels which I 
have already indicated in lulus * and Spirostreptus, as passing round the sides of the 
* Philosophical Transactions, 1841, p. 103. 
MDCCCXLIII. 2 O 
