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, Calcarella, Echinospira, etc., while Mac- 
