126 CLASS ACEPHALA. 
forms, but are never divided into four leaflets; the shell is re- 
placed by a cartilaginous substance, sometimes so thin that 
it is as flexible as a membrane. 
We form two families of them; the first comprehends the 
genera whose individuals are isolated, and without any 
organic connexion one with the other, although they fre- 
quently live in society. 
BipHora, Brug. THawia, Brown. SALPA, and 
DAGYSA, Gm., 
Have the mantle and its cartilaginous envelope oval or cylin- 
drical, and open at both ends. On the side of ‘the anus the 
aperture is transverse, wide, and furnished with a valvule, 
which permits only the entrance of water, and not its exit; 
on the side of the mouth it is simply tubular. Some muscu- 
lar bands embrace the mantle and contract the body. The 
animal moves by causing the water to enter through the pos- 
terior aperture, which has a valvule, and by making it issue 
through that on the side of the mouth, so that it is always 
pushed backwards, which has caused the posterior aperture 
to be taken by some naturalists for the true mouth. It also 
generally swims upon its back. Its gills form a single tube 
or riband, provided with regular vessels, placed obliquely in 
the middle of the tubular cavity of the mantle, in such a 
manner that it is constantly bathed by the water as it tra- 
verses that cavity. The heart, viscera, and liver are clustered 
up near the mouth, and towards the back, but the position of 
the ovary varies. The mantle andits envelope, when exposed 
to the sun, exhibit the colours of the rainbow, and are so 
diaphanous that the whole structure of the animal can be seen 
through them; in many they are furnished with perforated 
tubercles. The animal has been seen to come out from its 
envelope without appearing to suffer pain. The most curious 
circumstance respecting them is their remaining united for a 
