58 The American Naturalist. [January, 
the other. The Rotifers go back to the Trochophora, and Semper’s 
Trochosphera shows this relationship. 
The Annelids include Chetopods, Echiuriden, Dinophilus, Myzos- 
toma, Hirudinea, Branchiobdella. 
The treatment of the group is interesting, and brings out clearly 
the tendency of the authors to overestimate, as I believe, the value 
of embryology as affording a solution to phylogeny. The Trochophore 
larva is the typical larval form of the Annelids. It is exceedingly 
probable that the Trochophore of Annelids is a recapitulated stage of 
an ancestral form of the group which was common to the Annelids, 
Mollusca, and Molluscoida, and from which these groups diverged. 
It is difficult to determine from what ancestors the Trochophore itself 
arose. The authors give their reasons for rejecting the Medusz as the 
ancestors of the Trochophore. They conclude that the facts indicate — 
that the Trochophore developed directly out of a ciliated gastrula-like 
forefather, and by a change in the method of progression. The tran- 
sition of the Trochophore-like ancestor into the typical Annelid 
(Archiannelid) took place by an increase in the length of the body — 
and a diminution of the head. At the same time a change froma free- 
swimming to a crawling animal took place, and this process is repeated 
in the ontogeny of living Annelids. The Sipunculids show resem — 
blances to the Annelids, but cannot certainly be referred to that group; 
and their resemblance to Phoronis and Molluscoids is not sufficiently — 
established. The Chzetognatha are perhaps most nearly related to the 
Annelids. a 
The Tornaria of Balanoglossus shows certain resemblances to Echino- : 
derm larve, but it appears to be only in external and non-essential 
characters, and the Tornaria comes nearer to the Trochophore. There 
is some uncertainty as to the relationship of Balanoglossus to thë — 
rdata. o 
The radial structure of the Echinoderms is due to the ancestral form a 
having been fixed and subsequently become a free-living animal. To : 
what group of the Bilateralia these go back to is ‘‘in der Luft.” a 
cannot believe the authors have fairly treated the apparent relationship, — 
as Metchnikoff has pointed out, between the Echinoderms and ee 
glossus, through perhaps a Holothurian form.) The larva of the 
Echinoderms, they think, comes nearest to the T rochophore. ) 
Whether or not we agree with these more speculative parts oF © 
volume, the general verdict seems to be that the authors have p Be. 
a useful book. An English translation would probably appeal ys 
larger audience and excite a wider interest in embryology. 
