4 Prof. J.C. Schiddte on the Structure of 
These prolongations take rise from the sides only of the 
rings of the body—that is, from the pleure or lateral folds be- 
tween the dorsal and ventral plates; and in that case the 
pleuree may be covered to a greater or less extent by a de- 
posit of chitine forming a lateral plate, called the epimeron, 
which affords support for the muscles which move the basal 
joint of the limb, the hip or coxa. 
It is also from the pleuree alone that the peculiar prolonga- 
tions arise which are in the service of respiration, either out- 
side the body as gills or branchie, or inside as trachee. 
From the dorsal and ventral arches of the rings, on the con- 
trary, no limbs ever take rise, although they often carry other 
more superficial formations (which may also be subservient to 
locomotion) for the purpose of supporting or fixing the body, 
such as groups of sharp granules, indentations, spines, retrac- 
tile warts, often with hooks disposed in cireles, &c. ‘To this last 
class belong the so-called ventral or false feet of many larve. 
The cerci of Insects with perfect metamorphosis (both of imagos 
and of larvee) are merely transformed dorsal spines. 
The head consists, as all the other rings of the body, of a 
dorsal arch, a ventral arch, and two side pieces. In this case, 
too, it is merely from the side pieces that the limbs take rise, 
namely the appendages of the mouth, and, besides, the eyes and 
antenne. But as the head, besides one or two pairs of an- 
tenn, possesses three pairs of limbs (appendages of the 
mouth), whilst none of the other rings have more than one 
pair, the lateral pieces or pleure of the head (cheeks and 
temples) preponderate so greatly over the dorsal and ventral 
ieces, that they occupy the greater part of the skull, partieu- 
fatty behind and above, where so much space is required for 
the accommodation of the muscular and nervous systems; 
whilst the dorsal and ventral pieces lose so much the more in 
extent as they are here relieved from the office they perform in 
all the other rings (saving the last of all), viz. to afford sur- 
face of insertion for the muscles which move the next fol- 
lowing ring. The dorsal piece is therefore reduced to a plate 
above the mouth (the epistoma), from which separate pieces, 
viz. clypeus, labrum, labellum, are evolved in proportion as the 
mouth requires covering from above. The ventral piece, on 
the contrary (hypostoma, or the sphenoid plate, as it may be 
ealled from its analogy to the os sphenoideum of vertebrata), 
enters into closer relations with the appendages of the mouth, as 
will be shown more in detail by-and-by ; from this, too, sepa- 
- rate pieces are evolved in front, namely mentum and fulerum 
labii, in proportion as the maxille and lingua with their muscles 
require support and cover from beneath. 
