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 u 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, 

 as in some Turbellarian Avorms 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 eggs are fertilized, those of Laclnularla, however, 

 according to Huxley, not being impregnated. 



The eggs of Brachionus arc 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 cmite 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 Sag Ma, but the entire de- 

 velopment of the germ is much as in the mollusk Calyptrwa, 

 as Salensky often compares the earliest phases of devel- 

 opment of this Itotifcr with those of that mollusk. The 

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

 as a SAvelling 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. 123, v), and their relation to the head is as 



