VOL. 



9 X "] PROCEEDINGS OF THE NATIONAL MUSEUM. . 287 



in development, which calls for the maximum of function with the mini- 

 mum expenditure of tissue, and the other rule, which associates with 

 greatest vigor of life the most successful oxygenation of the blood, to- 

 gether with the obvious benefits to be derived from temporary protection 

 of the newly hatched larvce, will enable us to suggest an auswer. 



The prolongation forward of the siphonal septum, especially in forms 

 with short siphons, like Poromya and Lyonsiella, will evidently promote 

 successful aeration of the blood by cutting off from the branchial 

 chamber the water of the anal chamber, fouled more or less by the 

 effete products discharged into it. A certain amount of fibrous tissue 

 must be developed to form this septum. It is clear that an economy 

 of tissue would result from the transfer of retractorial functions to the 

 septum and the obsolescence of the lateral retractile musculature. A 

 further economy would result from the utilization of this large sheet of 

 tissue for branchial purposes, and a diminution of the tissue previously 

 expended in the mass of the ctenidia. The habit of the larvae, so com- 

 mon among Pelecypods, of nestling for safety in the branchial folds, 

 would lead directly to the utilization of the chamber as a refuge. But 

 a close chamber such as we see in Verticordia would, from the less pure 

 character of its contained sea-water, be less favorable than one into 

 which the water could be more freely admitted by any means which 

 would not imply an admixture of the foul water with that of the bran- 

 chial chamber below. A system of orifices like those of Myonera would 

 accomplish this. A subsequent development of the muscular tissue of 

 the septum, so that it could operate somewhat after the fashion of a 

 pump and voluntarily frequently renew the water in the anal chamber, 

 would obviously be beneficial. By the effect of stimulation the mar- 

 gins of the orifices thus subjected to repeated strong currents of fresh 

 water would be likely to undergo a specialization of respiratory func- 

 tions as compared with the rest of the surface of the septum, which 

 would result in something like the tubular gills of Cctoconcha, or the 

 lamelhe of Poromya. 



The gradual diminution of the ctenidia and increase of the area of 

 the siphonal septum is illustrated by such a series as Lyonsia, Lyonsi- 

 ella, aud Verticordia, all of which possess true ctenidia. 



The gradual specialization for branchial purposes of the septum after 

 the extirpation of the ctenidia would be illustrated by the series from 

 Myonera aud Cuspidaria to Cetoconcha and Poromya. 



While the above chain of hypothesis harmonizes with the observed 

 facts in a satisfactory manner, it is stated merely as a possible hy- 

 pothetical explanation, and not as a theory to which the writer must 

 staud permanently committed. 



General summary. — Premising that in this article the word ctenidium 

 is employed to designate the normal typical gill of Pelecypods, in any 

 of its modifications, as opposed to temporary or local branchial organs 

 of different origin, the facts just reviewed may be briefly summarized. 



