20 THE NAUTILUS. 



occurs in this place." Schereschewsky says, " Fertilization 

 takes place in the gill chamber." In the majority of the speci- 

 mens of Calyculina and Sphaerium ripe sperm and eggs were 

 found to occur in the same individual. None of the specimens 

 had eggs in the genital duct. 



Breeding Seasons. The breeding season probably continues 

 through the greater part of the year. Observations on this 

 point have been very meager. Animals taken in November and 

 December of 1913 were found to contain, in newly formed brood 

 pouches, eggs some of which were unsegmented and others in 

 very early cleavage stages. Considerably over fifty adult speci- 

 mens have been sectioned. All were found to contain young 

 in several stages of development. 



Gills. Before considering the structure of the brood pouch 

 it will be necessary to look into the structure of the gills. The 

 gills are four in number, an outer and an inner pair. The outer 

 is much smaller than the inner and falls short anteriorly by 

 about a fourth of its length. Each gill has two lamellae. The 

 outer lamella of the outer gill is attached to the mantle; the 

 inner lamella of the outer gill is attached to the outer lamella 

 of the inner gill and the inner lamella of the inner gill is at- 

 tached to the body. It is the outer lamella of the inner gill 

 which contains the brood pouches. 



The lamellae are made up of gill filaments (Figs. 7, 8, and 

 11). A typical filament may be compared to a rubber tube 

 sharply bent on itself to form a letter Y. Each filament of one 

 lameDa is therefore continuous with one of the other lamellae. 

 The open part of the letter Y represents the cloacal chamber. 

 All water which passes between the filaments finds its way into- 

 this chamber and from thence to the exterior. In the anterior 

 and posterior parts of the gill the cloacal chamber is very much 

 reduced (Figs. 8 and 11). 



Each filament is a hollow tube which in frontal section ap- 

 pears as an irregular ellipse (Figs. 8 and 11). The outer part 

 is made up of a single layer of heavy cells, strengthened by 

 chitinous rods (Fig. 8). The inner part is a single layer o£ 

 flattened cells forming a very thin membrane. The hollow 



