136 ZOOLOGY. 
The nervous system is very simple, consisting of a rather 
large ganglion situated behind one wing of the velum, and 
lying just under an eye-spot. A supposed organ of hearing, 
consisting of a sac filled with calcareous matter, is attached 
to the ganglion. 
The sexes are distinct, and the male and female reproduc- 
tive glands open into the cloaca. The sexes are, moreover, 
remarkably unlike, the males being much smaller than the 
females, rudimentary, sac-like in form, without any digestive 
sac, and are very short-lived. Some Rotifers produce what. 
are called winter as well as summer eggs; the former being, 
asin some Turbellarian worms and Polyzoa, covered with a 
hard shell to resist the extremes of the winter temperature. 
The summer eggs develop without being fertilized, while the 
winter eges are fertilized, those of Lacinularia, however, 
according to Huxley, not being impregnated. 
The eggs of Brachionus are attached by a stalk to the 
hinder part of the body of the female. The following 
remarks apply to the mode of development of the fe- 
male eggs, which are quite distinguishable from the mas- 
culine ones. The eggs undergo total segmentation, and 
the outer layer of cells resulting from subdivision forms. 
the blastoderm, and when this is developed the forma- 
tion of the organs begins. The first occurrence is an in- 
folding of the blastoderm (ectoderm) forming the primitive 
mouth, which remains permanently open, the mouth not. 
opening at the opposite end as in Sagitta, but the entire de- 
velopment of the germ is much as in the mollusk Calyptrea, 
as Salensky often compares the earliest phases of devel- 
opment of this Rotifer with those of that mollusk. The 
‘¢trochal disk,” or velum, arises in certain mollusks, 
as a swelling on each side of the primitive infolding. 
There is soon formed at the bottom of the primitive in- 
folding a new hole or infolding of the ectoderm, which is. 
the true mouth and pharynx, while a swelling just behind 
the mouth becomes the under lip. The stomach and intes- 
tine arise originally from the endoderm. 
Soon after, the two wings of the velum become well 
marked (Fig. 93, v), and their relation to the head is as. 
