COE: TERRESTRIAL NEMERTEAN OF BERMUDA. 
563 
side of the commissure. This outgrowth forms the lining of the 
proboscis, which is consequently of ectodermic origin. This ecto¬ 
derm grows backward as a solid mass of cells, which soon arrange 
themselves into a narrow tube lined with cuboidal or low columnar 
cells. Its growth posteriorly is accompanied by a multiplication and 
crowding together of the adjacent mesodermic cells, which later 
form the muscular walls of the proboscis. 
The tube with its cap of unarranged ectodermic cells grows back¬ 
ward toward the posterior end of the body in the median line, dor¬ 
sal to the mass of endoderm cells and immediately internal to the 
body walls. A section through the body at the point just reached 
by the end of the advancing proboscis (pi. 25, fig. 17) shows a solid 
mass of cells of which the central ones are probably ectodermic 
and the lateral ones mesodermic in origin. 
As the growth continues to the posterior end of the body the 
ectodermic lining of the tube of the proboscis remains closely 
invested with mesodermic cells, which form the muscular and con¬ 
nective-tissue walls. 
The mesoderm immediately surrounding the proboscis becomes 
separated from the adjacent mesoderm, the cleft forming the rhyn- 
chocoel. The adjacent mesoderm forms the proboscis sheath, 
which for a long time remains as a single layer of delicate fibers 
(pi. 25, fig. 18). It later becomes thickened and its musculature 
is arranged in definite layers as in the adult. In Careinonemertes 
(Coe, : 02) the separation of the proboscis from the proboscis sheath 
takes place only as far back as the anterior end of the posterior 
chamber, the walls of this chamber remaining permanently con¬ 
nected with the walls of the sheath. 
The development of the armature of the proboscis proceeds very 
much as in Prosorhochmus as described by Biirger (’ 95 , p. 482). At 
a definite point in the posterior third of the proboscis the muscular 
and connective tissues increase to form a thick transverse septum, 
which marks off the proboscis cavity into an anterior and a pos¬ 
terior chamber. These two chambers remain connected by a com¬ 
paratively narrow tube, which is at first lined with a single layer of 
regularly arranged columnar cells (pi. 25, fig. 20). On one side of 
this narrow tube a small evagination of the columnar epithelial 
cells forms a short pouch, which is without any lumen, consisting of 
an almost solid mass of columnar epithelium. These cells then 
