A. D. Imms 
107 
nervous system (Fig. 3). In the eighth and ninth segments, the dis¬ 
position of the longitudinal muscles is much modified, and many of the 
muscles come to lie nearer the digestive canal. 
Just in front of each nerve ganglion, a transverse band crosses 
the nerve cord, and passes along the line of junction of two adjacent 
segments (Fig. 22). 
In the eighth abdominal segment there is a special system of 
muscles in connection with the supporting skeleton of the spiracles. In 
order to attempt an explanation of these muscles it is necessary to 
briefly consider first the structure of this skeleton. From the posterior 
region of this segment of the body a median spiracular lobe, as it may 
be termed, projects backwards and very slightly upwards (Fig. 4). It 
stands out free from the body, and is supported basally by a special 
chitinised sclerite (sc.), which is prolonged on either side into a 
prominent plate provided with large backwardly directed teeth (pi.). 
The plates of either side are connected together by means of a trans¬ 
verse chitiuous band ( t.b .) which passes beneath the median spiracular 
lobe, and serves also to maintain the latter in a position so as to project 
out freely from the general surface of the body. The lobe itself consists 
of two thin lateral plates ( c.pl .) which have their free margins curved 
upwards and inwards, and a central, oblong plate of dark chitin with 
a curiously sculptured surface ( m.pl .). At the apex of the lobe the 
two lateral plates are joined together by means of a median trans¬ 
verse plate (t.pl.). The two spiracles (sp.) are situated at the anterior 
end of the median lobe near to the base of the latter and where it 
becomes joined to the segment. In front of and between the spiracles 
is situated a moveable transverse plate standing more or less at right 
angles with the general surface of the body ( fpl .). This plate is 
attached to a stout hollow peg of very dark chitin (p.) which projects 
beneath the integument slightly into the cavity of the animal. To 
this same peg is also attached the median plate (m.pl.) of the spiracular 
lobe. 
There are three paired sets of muscles in connection with the 
respiratory apparatus as follows: firstly, a pair of longitudinal muscles 
( l.musc .), which take their origin from the median peg (p.), and pass 
directly backwards to be inserted into the spiracular lobe, in the mid- 
ventral line near to the apex of that organ. Secondly, a series of 
muscles (i.mus.) arise from the ventro-lateral margin of the eighth 
abdominal segment and, pursuing a backward and inward course, they 
converge and pass between the two longitudinal muscles just referred 
