294 



TUP: DEVELOPMENT 



to see that it produces an organization in every respect identical 

 in relation with that of the Zouphytic type, and specially homol- 

 ogous with that of the Holothurians. 



MOLLUSCA. 



One of the most thorough and exhaustive investigations of 

 the embryology of this type is that of Lereboullet upon the 

 fresh-water snail Lymneus. This animal is not so very dis- 

 tantly removed in its organization from the common land snail, 

 Helix (p. 203, fig. 122) but that the latter may serve to illus- 

 trate the general relations of the organs of an adult snail, if we 

 wish to compare the young Lymneus with a mature organiza- 

 tion ; and this is the more allowable in the present case, because 

 our object is not to trace the development to any particular form, 

 but to show that its general results are in accordance with the 

 type to which the embryo belongs. 



The first change that occurs in the globular mulberry-mass, 

 after it has passed completely through its segmenting process, 

 is a flattening and then a hollowing of one side of it until the 

 once solid globule is metamorphosed into a hemispherical cup. 

 Soon, however, the mouth of the cup is narrowed by a mutual, 

 approximation of its opposite edges until they form a junction 

 along a straight line (fig. 194, a, m), excepting at one point, where 



an oblong aperture (m) is left. The aper- 

 ture is the future mouth, and the cavity 

 to which it leads is the incipient stomach, 

 whilst the line of junction (a) of the ap- 

 proximated sides of the cup indicates the 

 trend of the vertical plane which divides 

 the body into symmetrical right and left 

 parts. At the same time, the little globular 

 masses which originally formed the bulk 



a 

 Fig. 194. 



Fig. 194. Lymneiis stagnalis. Lamck. 150diam. One of the earliest stages 

 of development of an air-breathing aquatic snail, c. The yolk-mass ; 6, peculiar 

 yolk globules, or segment spheres collected about the incipient mouth (in) ; 

 m to a the longitudinal axis. From Lereboullet. 



