DEVELOPMENT OF NEMERTEANS. 
155 
both the sheath and proboscis lie between the commissures 
of the ganglia in the front part of the head. 
The ovaries and testes are situated in sacs 
on each side of the digestive canal. The 
sexes are distinct, with the exception of cer- 
tain species of Borlasia. The breeding sea- 
son is from March to April, while others 
spawn all summer. The eggs are ejected 
from lateral, pale, minute openings, and the 
species may be either oviparous or ovovivipa- 
rous. These worms when molested often 
break into fragments; in such cases each 
piece is capable of reproducing the entire ani- 
mal and all its internal organs. 
The Nemerteans present a great range of 
variation in their mode of development. In 
the simplest mode of growth the young is a 
ciliated oval form, without any body-cavity. 
In others there is a body-cavity, but the larva 
is minute and ciliated, and attains the adult 
form by direct growth. In still another spe- 
cies (Nemertes communis) the embryo is a 
ciliated gastrula, but leaves the egg in the 
adult form. ‘In others there is a complete 
and most interesting metamorphosis. In 
several Nemertean worms the egg undergoes 
total segmentation, leaving a segmentation- 
cavity. The next occurrence is the separa- 
tion of a one-layered ciliated blastoderm; the 
ectoderm, which invaginates, forming the 
primitive digestive cavity, from which the 
stomach and csophagus are formed. The 
larva (originally described under the name of 
Pilidiwm) is now helmet-shaped, ciliated, 
with a long lash (flagellum) attached to the 
posterior end of the body. (Fig. 106.) 
After swimming about on the surface of 
the sea a while, the Nemertes begins to grow 
Fig.105. — Pro 
rhynchus fluviatilis, 
one of the simplest 
Nemertean worms. 
o. mouth: @, ceso- 
phagus;i, intestine; 
1, glands opening 
into the intes- 
tine; 0, ciliated pits; 
2, 8tyle in the pro- 
boscis situated 
above the cesopha- 
gus, which ends ina 
blind sac at y ; ov, 
ovary, with eggs in 
different stages of 
development. The 
worm is externally 
ciliated.—After Ge- 
genbaur, 
out from near the esophagus of the Pilidium. On each 
