SOME POINTS IN THE ANATOMY OF THE PLATYDESMIDE. 509 
Silvestri. Here I must take the opportunity of correcting 
an error in my paper on the Skeat collection. The figure 
that I gave there as the mandible of P. kelantanicus is im- 
perfect. ‘he delicate chitin which forms a great part of the 
mandible has been torn away leaving only a part of the 
structure. j 
Fig. 14 shows the mandible of mexicanus. As will be 
seen in addition to the teeth and the small comb of bristle 
shown in Silvestri’s figure the clump of bristles marked Z in 
his figure are, in my species, much enlarged, and the bristles 
are arranged in a concave space in a definite order (figs. 14, 
15). Itis possible that the curves may be exaggerated by 
the shrinkage of the chitin, but I have examined a great 
number under the microscope and think that the arrangement 
shown in my figure is correct. 
Fig. 15 shows the two mandibles in situ on the hypostoma, 
and we can see that the effect of the configuration of the con- 
cave region in which the bristles are arranged is to form a 
sort of atrium set with bristles through which all food must 
pass before it reaches the gullet. In correspondence with 
this structure the muscles of the mandible are comparatively 
feeble, and only allow of a small amount of movement of the 
appendage. 
There is another point in the composition of the exoskeleton 
which deserves attention. ‘The suture passing along the back 
has been mentioned by Humbert and Saussure as only occur- 
ing in the Platydesmide and in Craspedosomide. 
When one cuts a section through the dorsal region one sees 
the appearance shown in figs. 11, 12, 13. The longitudinal 
division between the two parts of the body-ring is complete, 
and not only that but there is a narrow longitudinal plate 
intercalated between the two parts. A comparison between 
the two species of Platydesmusand Craspedosoma shows 
that the conditions are nearly similar. In both cases the in- 
tercalated plate does not extend the whole length of the seg- 
ment, but is only present in its anterior part and has the shape 
of a long narrow wedge. 
