THE NAUTILUS. 17 



Historical. Jacobsen (1828) noted the fact that embryos 

 of Cyclas develop in sacs. He observes that "each ovary is 

 composed of a number of small cylindrical sacs or capsules. 

 When impregnated, these sacs increase in bulk and gradually 

 protrude from the abdomen. By this protrusion they are intro- 

 duced into the gill cavity but still retain connection with the 

 interior membrane of the ovary. These capsules contain the 

 eggs and the young are developed in them. Each one contains 

 but one egg or young one. As soon as the young has reached 

 a certain size, the capsule bursts and the young is ejected into 

 the gill cavity. We find in the gill cavity at one and the same 

 time, capsules and young both large and small." Apparently 

 the only part of Jacobsen' s observations which is correct, is the 

 fact that the young develop in sacs. Later authors fail to con- 

 firm his work. Oskar Schmidt (1854) investigated the anatomy 

 of Cyclas ccdyculata. Franz Leidig (1855) studied the anatomy 

 of Cyclas cornea. Stepanoff (1865) was the first to point out 

 the brood pouch of Cyclas in its true relation. His observations 

 were confirmed in 1885 by Ziegler. However, the work of 

 both of these men was directed mainly toward segmentation and 

 development of the embryo and their observations on the brood 

 pouch were mere casual notes. In one of his plates Ziegler 

 shows a diagrammatic figure including a small portion of a 

 brood pouch. De Bruyne (1898) in a work on phagocytosis 

 figures a brood pouch. Poyarkofif (1910) published a prelim- 

 inary note on the incubation of embryos of Cyclas. This was 

 followed in 1911 by a paper on the same form by Schereschewsky. 

 Both of these authors gave considerable attention to the cellular 

 structure of the pouch, its origin and function. The only work 

 that has been published on American forms is that of Drew, 

 who in 1894 described the anatomy of Sphaerium sulcatum. 



Material. The material for this work was collected during 

 the summer and fall of 1913, from ponds and streams in the 

 neighborhood of Ithaca, N. Y. Two forms were observed, 

 Calyculina truncata and Sphaerium simile. 



Calyculina truncata is one of the smallest of the Sphaeriidae. 

 It averages about eight millimeters long, and seven high. The 

 shell is very fragile, rhombic ovate, the posterior part very 



