ANATOMY AND HISTOLOGY OF PLEUROCH ATA MOSELEYI. 487 
qui existe entre cette disposition et celle qui est propre aux Lombrics c’est que 
dans chaque groupe de faisceaux, ces derniers n’ont pas une orientation 
determinée et l’on ne retrouve plus par conséquent cet arrangement si regulier 
qui frappe tout de suite chez ces derniers animaux.” I have attempted to show, 
in the preceding description of the muscular layers of Plewrochwta, that there 
are a series of transitions between the muscles of Lwmbricus and those of other 
forms; this series is not very complete, but it serves to indicate that there is 
nothing really peculiar in the muscles of the former. 
Sete are found in great numbers in Pleurochwta, and their characteristic 
distribution has been already treated of. They are in shape like the ordinary 
forms of setze among the Oligocheeta, that of an elongated S ; in Pontodrilus we 
have apparently the only exception to this rule; in that genus the sete are 
straight and rod-like. The length of the sete of Pleurocheta vary from 
035 mm. to (066 mm.; they are therefore rather small in comparison with those 
of other Oligocheta ; in Pontodrilus, for example, the length is about ‘16 mm., 
and the largest sete of Lwmbricus that I have measured myself are from ‘1 mm. 
to 12 mm. Plate XXVI. fig. 18 shows one of the sete in position. The 
section is taken from the anterior part of the body; the seta, which is of the 
ordinary amber yellow tint, lies in a diverticulum of the outer chitinous 
layer of the integument, and extends downwards through the circular muscu- 
lar layer and someway into the longitudinal (@ marks the boundary of 
the two coats); towards the middle of the seta there is a slight swelling, 
which appears to contain spaces filled with air; this swollen part is developed 
about the end of the upper third of the seta; in other worms its position 
is rather different ; it appears to be fairly central in the Pericheta described 
‘by Leon Varriant.* In Urochwiat it is rather nearer the upper end, and also 
in Lumbricus. 
The free extremity is curved rather sharply, the opposite end being rounded 
off, and very faintly bifurcate. The texture is uniform, except for the presence 
of faint wavy transverse markings and longitudinal striations. The arrangement 
of the setze and the muscles which move them present no important differences 
from other Oligocheta. The seta is enclosed in a “ cul-de-sac” formed by a 
fold of the cuticular membrane. Each is provided with its special muscles, 
which are inserted into the cuticular covering of the seta at one end, and at 
the other some of them appear to be inserted close to the hypoderm, and 
others to form part of a continuous layer of muscles connecting the several 
setze of one segment, while others again seem to join the outer or transverse 
muscular layer of the body wall. These special muscles of the seta appear 
to be arranged in about six bundles, radiating outwards from the cuticular 
* VAILLANT, loc. cit. 
+ Porrrisr, loc. cit. 
