178 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 unlilie, the males being much smaller than the 

 females, rudimentary, sac-like in form, without any digestive 

 sac, and are very short-lived. Some Eotifers produce what 

 are called ivinter as well as sumvier eggs ; the former being, 

 as in some Turbellarian ivorms and Polyzoa, covered with a 

 hard shell to resist the extremes of the winter temperature. 

 The summer eggs develop withoi^t being fertilized, while the 

 winter eggs are fertilized, those of Lacinularia, however, 

 according to Huxley, not being impregnated. 



Tlie 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 tlio blastoderm (ectoderm) forming the primitive 

 mouth, which remains permanently open, the mouth not 

 opening at the oj^posite end as in Sagitta, but the entire de- 

 velopment of the germ is much as in the mollusk Caly^Jtrma, 

 as Salensky often compares the earliest phases of devel- 

 opment of this Eotifer with those of that mollusk. The 

 "trochal disk," or velum, arises in certain mohusks, 

 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 

 tlie true mouth and pharynx, while a swelling just behind 

 the mouth becomes the under lip. The stomach and intes- 

 tine arise originally from the eudoderm. 



Soon after, the two wings of the velum become well 

 marked (Fig. 133, v), and their relation to the head is as 



