140 TECTIBRANCHIATA.—APLYSIAD A. 
large vessel which collects and receives the venous 
blood from all parts of the system, and carries it 
forward to the gills, was believed to have nume- 
rous perforations in its walls, through which a free 
communication existed with the general cavity of 
the abdomen, so that the fluids contained in the 
one could readily permeate the other.* But it has 
been since proved that these supposed perforations 
are merely depressions, and that the lining mem- 
brane of this great blood-vessel is entire, as in other 
animals. 
The Mollusca of this Order undergo a metamor- 
phosis exactly similar to that already described in 
the Nudibranchs; there is, in fact, scarcely any 
appreciable difference in the form of the newly- 
hatched young in either of these Orders, in that 
of the Pectinibranchs, and in the Class Pteropoda. 
How long the infant animal remains in this, its 
first condition, is not yet ascertained. Arrived at 
the second stage, we find it still enclosed in its 
transparent and nautilus-like shell; but the mantle 
has become detached, and covers tightly the mass 
of the viscera. The foot is so enlarged, that it 
forms a considerable projection beyond the margin 
of its operculum ; and the veils have also grown 
in size, while the eyes have altered to a violet 
colour. The head has now two short, conical, 
ciliated tentacles, and the little animal swims 
with surprising quickness. 
In the third stage the shell has fallen off, and the 
general shape is that of the parent, but the veils 
still remain. In the fourth stage, the creature 
begins to crawl in the gasteropod fashion, and the 
branchie and ceca begin to sprout. ‘There are now, 
* Mem. sur les Mollusques, ix. 14. + Grant’s Comp. Anat. 465. 
