EMBRYOLOGY OF BDELLODRILUS 201 
During the last thirty years embryologists have differed in 
their conception of the origin of the mesoderm and. of its phylo- 
genetic significance. Hatschek (’78) was among the first to 
distinguish between. mesenchyme and mesoderm, but held, after 
studying the embryology of Polygordius, Echiurus and Eupo- 
matus, that these two morphologically different mesoblasts, 
arise from a common foundation. This same view was later 
put forth by the Hertwigs (’81) in their ‘Coelomtheorie,’ which, 
according to Meyer, has formed the foundation of all later work 
on mesoderm. Roule (’89 and ’94), Burger (’91 and 794), Frai- 
pont (’88), Hacker (’95), and others, have described the meso- 
blast as having a single origin. On the other hand, those who 
have studied the embryology of annelids and molluscs, con- 
sider the origin of the mesenchyme distinct from that of the meso- 
blast or coelo-mesoblast. This later view was first described 
by Kleinenberg (78 and ’86), and later by Whitman (’87), 
by Berg (’90), by Schimkewitsch (’94), by Meyer (01), by 
Torrey (03), and others. 
A larval mesoblast was first described by Lillie (’95) in Unio. 
It arises asymmetrically from the derivatives of a? and later mi- 
grates into the segmentation cavity, where it divides equally 
and becomes symmetrically arranged on either side of the mid- 
line. The derivatives of these two cells become metamorphosed 
into ‘myocytes’ and larval adductor muscles, which are functional 
only during larval life. 
Treadwell (97) regards both mesenchyme and mesoderm 
as morphologically the same tissue, the apparent difference 
in their mode of origin being of no significance. And, further, 
Wilson regards the larval mesoblast (ecto-mesoblast, because 
of its origin from the ectoderm) as a distinct tissue from that 
of the definitive mesoblast or ento-mesoblast, and states that it 
is homologous with the mesenchyme of the turbellarian ances- 
tors of the annelids, while the mesoblast from which the adult 
structures arise is phylogenetically younger and is represented 
prophetically in the ontogeny of such a form as Discocoelis 
(polyclade) by the peculiar lateral division of M, and states that 
the ecto-mesoblast and endo-mesoblast are phylogenetically of 
