450 BULLETIN OF THE 



that the two series represent a parallel among these Pelecypods to the ctenidia 

 and the peripedal laminae in Acmcea, Scurria, and Patella, among the Docoglossa. 

 That is, that the septal laminae are a new and special development, which func- 

 tionally replace, but are not homologous with, the original ctenidia. If this 

 view is doubted, the burden of proof lies upon those who call it in question. 



It may be asked whether any hypothesis can be suggested by which this 

 peculiar specialization may be accounted for. The law of economy in de- 

 velopment, which calls for the maximum of function with the minimum 

 expenditure of tissue, and the other rule, which associates with greatest vigor 

 of life the most successful oxygenation of the blood, together with the obvious 

 benefits to be derived from temporary protection of the newly hatched larvae, 

 will enable us to suggest an answer. 



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 muscula- 

 ture. A further economy would result from the utilization of this large sheet 

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

 pended in the mass of the ctenidia. The habit of the larva?, so common 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 branchial 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 margins of the 

 orifices thus subjected to repeated strong currents of fresh water would be 

 likely to undergo a specialization of respirator}'' functions as compared with the 

 rest of the surface of the septum, which would result in something like the 

 tubular gills of Cetoconcha, or the lamellae of Poromya. 



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

 siphonal septum are illustrated by such a series as Lyonsia, Lyonsiella, and 

 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 

 and 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 hypothetical explanation, 

 and not as a theory to which the writer must stand permanently committed. 



