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 Ostreadee, the sexes are distinct : if this 

 be so, how can the genial influences pass ? for we must pre- 

 sume some sort of coiigression. 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 coiigression, 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 

 ciliated 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 Ascidice. 

 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 light 

 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 

 opercida of the Balani, which are imbedded, with a limited 

 movement, in a cartilaginous 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 



