Aplysia calif ornica — Winkler 
349 
PLATE 
membrane 
MITAMORPHIC SHELL 
Fig. 2. Two views of newly hatched veliger larva. 
The measure indicates 150 ju. 
Fig. 1 . Nuclear area of a shell of Aplysia californica 
Cooper showing the stages of development from the 
microscopic free swimming veliger to the adult plan of 
shell construction. The measure indicates 150 fx. 
As the shell completes its first convolution, 
it starts an even more marked tendency to 
flare out and flatten to form a covering over 
the animal rather than a house into which it 
may retract. At the end of the first whorl this 
tendency has become dominant, and by the 
end of the further half whorl the shell has 
opened out into a spoon-shaped covering 
which is indicated as the late metamorphic 
shell in Figure 1. Mazzarelli (1893) pictured a 
metamorphic animal which is probably a 
somewhat later form than this, carrying its 
outgrown shell on its back. During the meta- 
morphosis, additional material is laid down 
on the shell, in which process the nucleus is 
probably first a point of muscle attachment. 
However, in A . californica an auxiliary plate is 
soon formed, paralleling the curvature of the 
intermediate metamorphic shell, which, along 
with later modifications, forms a new post- 
metamorphic plan of growth. This neaplysiid 
plate forms on a plane with the embryonic 
nucleus and becomes an extension of it. 
The entire outline of the shell is modified 
after metamorphosis, the original larval shell 
being used only as a building block in the 
future adult shell. These shell changes are 
indicated in Figure 1 by the lines of growth. 
SUMMARY 
1. The shell nucleus, containing the veliger 
shell, is present in adults of Aplysia californica 
Cooper as a building block in the auxiliary 
plate characteristic of this subgenus. 
2. The metamorphic forms of the shell are 
clearly preserved. 
3. The implications of this shell metamor- 
phosis are (a) a short swimming stage, (b) an 
intermediate, gastropodlike crawling stage in 
which the shell is a place of retreat, (c) a 
metamorphic stage with a spoon-shaped shell 
by which the animal is covered, and (d) a 
late metamorphic stage in which the animal 
outgrows the shell, which becomes vestigeal 
in the adult animal. 
REFERENCES 
Carazzi, Day. 1905. L’embriologia dell 
’ Aplysia e i problemi fondamentali. Arch. 
Ital. de Anat. 4:231-305. 
Mazzarelli, G. 1893. Monographia della 
Aplysiidae del Golfo de Napoli. Soc . Ital. 
Modena ., Mem. Ser. 3, 9:1-205. 
Saunders, A. M. Carr, and Margaret 
Poole. 1910. Development of Aplysia punc- 
tata. Jour. Micros. Sci. 55:497-539- 
