1885.| Embryology. IIIQ 
post-anal telson or bristles, and the preoral labrum and one or 
two pairs of antennæ may be supposed to have been derived 
from. a postanal, and a preoral series of tentacles respectively, 
supposing of course that the mouth is formed from the anterior 
part of the archistome, while the anus is formed from its posterior 
portion, while, as supposed by Sedgwick, the middle portion has 
coalesced. 
The biramose legs of Crustacea and certain insects may be 
supposed to have arisen from a bilateral actinozoan type in 
which there were two rows of tentacles encircling the oblong 
archistome. When the inner and outer archipodia of one side, 
as we may name these primitive limbs, had fused at their bases, 
we would have a biramose appendage. As the outer layer be- 
came chitinized these appendages would become segmented. A 
very primitive type of limb, which may be supposed to have been 
derived from the tentacle of an actinozoan ancestry, is found in 
Peripatus. The parapodia of worms may also be supposed to 
have been derived from two such circles of archipodia which 
surrounded the archistome, but which, as the body became 
elongated, assumed a more and more lateral position. A new 
set of structures are, however, developed in the parapodia of 
errant marine worms, the analogues of which are found only in 
the fin-folds of the embryos of osseous fishes, or as the rays of 
the most primitive and undegenerate types of adult forms, 
namely, the Elasmobranchii, Holocephali and Dipnoi. These 
structures are the sete which are of epidermal origin in the 
worms, or at most subepiblastic; as in embryo fishes and in 
Sagitta. In a former number of this journal I have called these 
structures in fishes actinotrichia; these are the same as the em- 
bryonic fin-rays mentioned by A. Agassiz. 
The principal reason why I consider the actinotrichia found in 
fish embryos analogous if not homologous with the’setz found in 
the appendages of worms, is the fact that in both cases muscular 
processes of the mesoblastic somites first become attached to the 
inner ends of these fine horny or chitinous filaments, which in 
the worms protrude beyond the margins of the soft tissue of the 
parapodia, but which in embryo fishes and in Sagitta do not ex- 
tend beyond the edges of the fin-folds. It is thus rendered ob- 
vious that bundles of muscular fibers derived from the muscular 
somites, developed from lateral gut-pouches, pass outward and 
are inserted upon the proximal ends of the sete found in the 
parapodia of worms as well as the actinotrichia found in the fin- 
folds of fish embryos. In fishes these muscular processes are 
given off to the actinotrichia of the unpaired as well as to those 
of the paired fins. These muscular processes moreover pass 
outward into epiblastic folds in both cases metamerically or from 
each segment. In the worm toa bunch of sete in a single para- 
podium, in the fish to a bunch or longitudinal series of actino- 
