OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
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small chambers of the heart in the last dorsal, or preanal segment of the body (20, 
21), but one only in each of the other segments. The posterior of these is of a pear- 
shaped form, and has four minute vessels at its extremity. The two middle ones are 
given to the posterior pair of legs, and the two outer pass backwards and upwards to 
the sides of the segment. The second chamber is short, and scarcely longer than 
broad. The other chambers are equal in length to that of their respective segments. 
The two posterior chambers, perfectly distinct from each other, but located in the 
same segment, are analogous to the two chambers which exist in each segment in 
the earlier periods of development, as we have already seen in the Iulidae. The ge- 
neral form of each chamber of the heart (figs. 20. 21.) is very peculiar. It is dilated, 
and somewhat lobular and rounded at its posterior extremity ( b ), but is narrowed 
towards its middle portion, and again enlarged at its anterior (f), but is very narrow 
and constricted at its junction with the next chamber (e). This constriction in the 
young animal, in which I have most carefully examined these structures, is only in 
the external tunic of the organ, and is not extended to the interior. The auricular 
orifices ( d ) through which the blood enters, are situated at the point of junction of 
this part with the next chamber. They are two somewhat elongated oval apertures 
(fig. 21. d), in the external muscular coat of the organ, placed a little diagonally on 
each side of the median line, on its dorsal surface, and are divided only by a thin 
muscular partition. They are bounded externally by a series of curved fibres (e), the 
continuations of those which connect the posterior of one chamber to the anterior of 
the one next behind it. These orifices, when examined in specimens that have re- 
mained long in spirits, often appear bag-shaped and dilated, and the fibres that 
bound their external margin are then very distinct. They seem very much to re- 
semble the apertures in the heart of Crustacea, and are, I believe, in their natural 
state continuous with the parietes of some exceedingly delicate venous trunks, that 
convey the blood to the heart. At the posterior part of each chamber, in the median 
line, on the dorsal surface, and extended backwards to the commencement of these 
orifices, there are a pair of suspensory muscles that pass diagonally upwards and 
backwards to their attachment in the next segment. These muscles still exist in the 
larvae of insects, and appear to have considerable influence on the auricular action of 
the chambers, and the admission of the blood. At a corresponding part of each 
chamber, on its under and lateral surface, there are two large arterial trunks ( b , c), 
which have already been mentioned in the Iulidoe as the systemic arteries . The dis- 
tribution of these, which supply most of the blood to the viscera and sides of the 
body, I shall presently describe. The alee cordis (figs. 18, 19._/) resemble those of 
lulus . 
The minute anatomy of the heart (fig. 22.) is exceedingly interesting. This organ 
is composed of two distinct contractile tunics, an external (a) and an internal one (e), 
each covered by its proper serous membrane. The external tunic is covered by the 
membrane that forms the inner layer of the pericardium. It is a very thick, muscular 
