ACEPHALA PALLIOBRANCHIATA. 21 
beyond acquisition; the only chance is, that a fortunate hit 
may be made, and these rare productions brought to light 
from the deep sea beds. 
That the animals of this tribe are of inferior organization 
to the lamellibranchiate bivalves, is, I think, shown by their 
invariable fixity of habitat. By-the-by, it is said that in this 
family, and also in the Ostreade, the sexes are distinct : if this 
be so, how can the genial influences pass? for we must pre- 
sume some sort of congression. Is it contended that the 
germs of vitality are committed, at hap-hazard, to the waves ? 
I cannot concur in this doctrine. I believe all the bivalves 
are strict hermaphrodites, that is, without congression, and 
contain within themselves the elements of bisexuality. 
I return from this digression to make a few more observa- 
tions on the value of the palliobranchiate organization. In 
addition to the fixed state, it may be added, that in it the 
circulation is more simple, the motive power consisting only 
of an auricle on each side, without a ventricle. The spiral 
ciated arms, though not articulated, in consequence of their 
hollow structure producing an equivalent flexibility, are to me 
sufficient evidence of an alliance with the Cirripoda; and I 
think the structure of the respiratory apparatus equally con- 
clusive that these animals are closely related to the Ascidie. 
Let us examine the upper valve of Crania, fixed as it is to the 
animal only by muscles. I can consider it in no other hght 
than as the analogue of the operculum of the Balani, for the 
protection of the animal, and support and regulation of the 
issue of the long, spirally-coiled, ciliated, tentacular buccal 
appendages ; and I think that the longer or shorter internal 
opercula of the Balani, which are imbedded, with a limited 
movement, in a cartilagmous mass, are only tantamount to 
the internal apophysary processes of such of the Pallio- 
branchiata as have them; and for those which are destitute of 
these peculiar supports, the issue of the long spiral arms is 
regulated, as in the pedunculated Cirripoda, which have not 
the moveable opercula of the Balani. The spiral arms in both 
species of Balani, as well as in the Brachiopoda, besides tenta- 
cular and buccal functions, may also serve as the muscular 
