_ ular ball and socket joint, but with a flexible part of th 
92 The American Naturalist. [February, 1 
consequently a deposit of calcareous matter took place produc- 
ing the stiffening aboye hinted at. 4 
The formation of jointed appendages from parapodic pad- 
dles of the annelides can be followed out in the same manner, 
since the manner of mutual relation of the segments is the 
same as in the case of the body segments. = 
It has been stated that in the leech the folds do not corre- 
spond in number to the somites of the body, while they do in : 
the crustacea. Allannelides do not move by means of a mus- 2 
cular system built upon the plan found in the leech. In ~ 4 
many the circular layer has to a large extent disappeared, | 
for the longitudino-circular plan is undoubtedly ante-annelit 
dan. The movement of the free medusoid forms, and of the — 
Ctenophora, is the result of a modified arrangement of this 4 
lan. i 
With the disappearance of the circular layer, we, find a 4 
peculiar modification of the longitudinal layer. This layer 
becomes broken up and the fibers act in moving the setæ, 
which answer to limbs. In a segment of a setiferous i 
annelid, we may observe that the longitudinal muscles of the i 
somite in section at the position of the seta are arranged like 
the letter “ V,” in the fork of which the seta lies, the fibers to 
the left (anterior) pull the seta externally backward, those on — 
the right (posterior) pull the seta forward. The introduction — 
of the sete, the origin of which I do not here attempt to — 
explain has no doubt been, together with the establishment of — 
external segmentation, a strong factor in causing the breaking © 
up of the muscular tube into sections (myotomes), which by — 
use and consequent increase have extended each arm of the 
“ V” into the segment on each side, while the insertion of the 
end of the seta has caused a break in the muscle by the for- 
mation of an aponurosis. This gives us the peculiar disposition 
of a myotome to extend arcoss the union of two somites. 
If we examine the segments of the so-called abdomen of the 
macrurous crustacea, as the lobster, we will find that the 
anterior face of one abdominal ring is pulled into the posterior 
orifice of the ring lying anterior to it, forming a kind of eer : 
ET ey TE oe Y 
Fehon 
te eo gee ee Ta eet Ohare 
integument with no calcareous deposit, folded upon itself, 
