REPRODUCTION 93 



known as the "spiral cleavage," and that this 

 obliquity takes a reverse inclination in sinistral forms 

 to that which obtains in dextral. 



In the succeeding stage of the development the 

 macromeres, which have undergone further sub- 

 division, form the inside lining of the mulberry, as 

 it were, and enclose a cavity which ultimately 

 becomes the digestive tube. Other cells arising 

 between the layers of micromeres and macromeres 

 subsequently form the various organs and muscles 

 of the young animal. Meantime there arises a 

 swimming organ called the " velum." It begins as 

 a ridge fringed with fine hairs (cilia) encircling that 

 part of the head of the embryo which lies in front 

 of the mouth, and it finally expands into a sort of 

 disc, which is drawn out into lobes or into fingers. 

 This condition is the "veliger stage," which, as 

 already remarked, is characteristic of the Mollusca 

 (Plate XXIX., Figs. 1-8). The stage is further 

 marked by the formation of the young shell and the 

 first appearance of the foot. As the young mollusc 

 grows up the velum disappears. 



Many larval forms, especially among the Gastro- 

 poda, are very different from the adult both in the 

 appearance of the animal and the shell, and have 

 been described as distinct species, while the adult 

 form of some has not yet been ascertained. Thus 

 the young shell of Lamellaria has received the names 

 of Brownia, Cakarella, Echinospira, etc., while Mac- 



