Nidificatio)} (Did Eiiibnjological Development 33 



[3] HeXisoma duryi noymalt Pilsbry 



Specimens of this planorbid, collected on the Tamianii Trail, forty miles west of 

 Miami, Florida, were sent to the writer by Mr. W. F. Shay of the Department of 

 Science, Normandy High School, St. Louis. They were placed in aciuaria in January, 

 1931. On January 31 several egg capsules were observed, one containing fifteen em- 

 bryos in various stages of development, three in veliger stage and twelve with well- 

 formed shells. B.y February 2, eleven snails had been hatched and were crawling on 

 the glass sides of the aquarium. By February 6 most of the veliger larvae had been 

 hatched. Another capsule contained fifteen eggs with one veliger stage, one gastrula 

 stage and thirteen embryos with well-formed shells. These latter hatched on Feb- 

 ruary 4. Another capsule contained ten eggs, five embryos with partly formed shells 

 and fi\e embryos with fully formed shells. These were hatched by February 5. 



On September 1, 1931, three capsules were observed on the side of the aquarium, 

 one with nine eggs, 7 by 4.5 mm. in area, one with ten eggs, 6 by 5 mm. and one with 

 eight eggs, 5.7 by 3.2 mm. On September 12, two additional capsules were deposited, 

 one with seven eggs (6 by 3.2 mm. in area) and one with four eggs (4.7 by 2.7 mm.). 

 Of the first series of three capsules, two died and were eaten by Protozoa and one 

 completed its development. Of the second set, one capsule carried out its full de- 

 velopment. In this capsule, the veliger stage was reached on September 15 and the 

 embryo with a well-formed shell on September 19. On September 20, the four snails 

 were actively feeding in the egg capsule, the walls of the egg capsules were broken, 

 and the snails were crawling about in the broken interior. On September 21, the snails 

 had left the egg capsule and were feeding on the side of the aquarium. It is probable 

 that this capsule had been laid for a day or two before first observed because the 

 interval of nine or ten days seems too short for comjilete development. 



On January 17, 1932, a recently laid capsule containing sixteen eggs was discovered 

 and was obser\ed until all of the embryos had been hatched (see plate 42, fig. 12). 

 In this cajisule fifteen days elapsed between early segmentation and exit from the egg. 

 By the middle of February the young snails measiu'ed 1.1 by 1 mm. and actively 

 glided about the aquarium. The body in the shell is pinkish, the foot hyaline, slightly 

 yellowish, the liver brownish, and the eyes black and prominent. The tentacles are 

 long and mobile. The radula can be plainly seen at work and the jaw appears blackish 

 through the translucent head. 



From the aboA'e capsule and from several others hatched at about the same time, 

 forty young snails were released in February. Observations were made on the shells 

 several times in March and April. On May 22 there were seven living snails and 

 thirty-three empty shells, indicating that mortality had been heavy, principally during 

 the month of May. The seven living snails had the following shell measurements: 

 1 specimen 1.5 mm. in diameter with 2 whorls. 

 1 specimen 2.5 mm. in diameter with 2^-2 whorls. 

 1 specimen 2.8 mm. in diameter with 3 whorls. 



1 specimen 3.4 mm. in diameter with 2->4 whorls. 



2 specimens 3.5 mm. in diameter with 3 whorls. 



1 specimen 4 mm. in diameter with 3% whorls. 



On June 26 the seven snails were still alive. By August 15 one had died. The six 

 snails gave the following measurements: 



2 specimens 4 mm. in diameter with 3 whorls. 



2 specimens 4.5 mm. in diameter with ZYi whorls. 



1 specimen 5 mm. in diameter with 3^4 whorls. 



1 specimen 5.5 mm. in diameter with 3^/4 whorls. 



On December 26, 1932, but four snails remained alive. These showed the meas- 



vu'ements below: , • . • i- , vu o u i 



1 specimen 4 mm. m diameter with 3 whorls. 



1 specimen 5 mm. in diameter with 3^/4 whorls. 



1 specimen 5.2 mm. in diameter with 3'!4 whorls. 



1 specimen 6 mm. in diameter with 3^/i> whorls. 



On March 12, 1933, three snails were living which mea.sured as follows: 



1 specimen 4.8 mm. in diameter with 3 whorls. 



1 specimen 5.7 mm. in diameter with 3Vi whorls. 



1 s])eciinen 6.1 mm. in diameter with 3Vi! whorls. 



