Brooks.] 228 [February 2, 



branchs are higher than or derived directly from the Gasteropods, 

 for any such conclusion is rendered impossible by the lack in the 

 latter group of such peculiarities as the lingual ribbon, a centralized 

 and highly evolved nervous system, and accessory organs of repro- 

 duction. Although it is true that these features might have been lost 

 through adaptation to a sedentary life, their entire absence at all 

 stages of growth, throughout the whole class, would seem to indicate 

 that they never existed; so we cannot derive these animals directly 

 from the Gasteropoda, but must regard them as an offshoot from a 

 form of which the Gasteropods are the highly developed linear or 

 nearly linear descendants. If this conclusion is accepted it is plain 

 that all attempts to trace the phylogeny of the higher Mollusca 

 through the Lamellibranchs to the Molluscoida, must be erroneous 

 and useless. 



The history of the discussion of the affinities of the Mollusca is an 

 almost unbroken record of generalizations based upon imperfect 

 knowledge and erroneous conceptions, and so many arrangements of 

 the group have been proposed, accepted for a time, and then shown 

 to be unnatural, that it is not at all strange that many naturalists 

 should now call in question the existence of any real affinity between 

 the higher and the lower classes. As long as the attention of the 

 investigator was confined to the study of shells, there seemed to be 

 no difficulty in connecting the Lamellibranchs with the Brachiopods 

 through such forms as Anomia; and although the slightest anatomical 

 knowledge is sufficient to show that the resemblance between these 

 forms is entirely superficial and without scientific value, this concep- 

 tion had been so generally accepted and so firmly established that the 

 confirmation by embryology of the results reached through anatom- 

 ical research, has scarcely been able to thoroughly exterminate it. 



This view has been replaced by another which is not open to the 

 charge of superficiality, since it is based upon a thorough knowledge 

 of adult structure, and its weakness is shown only when it is tested 

 by embryology. The clearest and most forcible statement of this 

 view is that given by Allman in his " Fresh-water Polyzoa." Accord- 

 ing to Allman the Tunicata are intermediate between the Polyzoa 

 below and the Lamellibranchs above. The branchial sac of a Tuni- 

 cate represents the permanently retracted tentacular crown of a 

 hippocrepian Polyzoon; the tentacles form the horizontal bars of 

 the sac, and uniting to each other at intervals inclose the branchial 

 slits. Although Allman's figures are necessarily diagrams, no organ 



