1148 The American Naturalist. [December 
ted genital tubes into two rows of equal segments. In this pro’ 
cess, very probably, certain muscular regions, especially those 
parts of the transverse and dorso-ventral systems serving in that . 
mode of motion, took active part during their contraction by 
continually constricting the undivided genital glands. 
The series of successive sexual glands thus arising, and ar- 
ranged symmetrically on both sides of the intestinal canal for the 
maintenance of equilibrium, now furnished internal metameric 
centres, about which the remaining organs, which were previously 
diffusely distributed in and on the body, also grouped themselves 
metamerically. This I regard as having taken place as follows: 
In the gradual increase of thickness and firmness of the skin, or 
perhaps merely from the secretion ofa cuticula of little elasticity ; 
and again in consequence of serpentine swimming movements, cir- 
cular furrows were formed upon the surface, where the integument 
wasthinner. The sexual follicles, swelling at maturity, and so 
distending the body at equal intervals, would predetermine defi- 
nite places for occurrence of these furrows,—that is, between two 
successive pairs of sexual glands. In the segmented body-zones 
thus marked out each somewhat centrally placed pair of the re- 
maining organs acquired greater perfection, and thus rendered all 
its homologues superfluous in its segment, so that these gradually 
underwent complete degeneration. This, in my opinion, was the 
origin of metamerism.® 
The cavities of the parenchyma probably first united into a 
large sinus about the intestine, from which lymph penetrated into 
the interior of the paired metameric sexual glands to nourish the 
developing sexual products floating in them. As this increased 
in quantity the follicular cavities expanded more and more, and in 
this way became transformed into the secondary body-cavity with 
its paired, segmentally arranged chambers. In the epithelial 
walls only certain areas, subsequently the sexual glands proper 
. § While writing this the recent “ Lehrbuch der vergleichenden Entwicklungsgeschichte 
der Wirbellosen Tiere '' of E. Korschelt and K. Heider came into my hands, and in it 
also, the “ serpentine motions” gave rise to such a process, since they must have led to 
the formation in the body of “ alternating regions of greater and less mobility." 
