1876.] 229 [Brooks. 



is exaggerated or suppressed for the purpose of making tbe likeness 

 more forcible; they are very accurate and faithful representations of 

 the animals, and show the closest similarity between these two forms; 

 the position, structure and connections of almost every organ of the 

 one being duplicated in the other. An almost equally perfect com- 

 parison may be made between a Tunicate and a Lamellibranch, but 

 the recent great additions to our knowledge of the embryology of 

 the Tunicata seem to show, with absolute conclusiveness, that we here 

 have nothing but a very perfect and striking adult resemblance, 

 reached in each of the groups in a different way and therefore with- 

 out homological signification. Whatever view of the vertebrate 

 affinity of the Tunicata we may incline to, we must recognize the 

 fact that the branchial sac is morphologically part of the digestive 

 tract, and in no sense whatever a lophophore or a tentacular gill. 

 Moreover we should expect, according to all analogy, to find the 

 affinity to other groups most clearly shown in the low or embryonic 

 forms, but Appendicularia presents none of the peculiarities upon 

 which the comparison is based. As Ray Lankaster has lately referred 

 to Allman's homology in a way which seems to imply that he still 

 accepts it, I will repeat more briefly my reasons for rejecting it. 

 These are : first, that the development of the Tunicate shows that 

 the resemblance is not due to community of origin, but is reached in 

 different ways : and secondly, that the adult Lamellibranchs are a 

 specialization of the embryonic type and therefore cannot lie in the 

 direct line connecting the Molluscoida with the Mollusca. Allman 

 himself seems to have seen the force of the first objection, for in a 

 much later paper (1869), he advances the view that the Polyzoa are 

 connected, through Rhabdopleura, with the Lamellibranchs. His 

 studies of this genus were made upon alcoholic specimens; and Sars, 

 who enjoyed the superior advantages afforded by an abundance ot 

 living specimens, has shown that Allman was mistaken in regard to 

 almost every one of the points upon which he attempted to establish 

 the supposed relationship. 



These are only a few of the arrangements of the Mollusca which 

 have been proposed, and the fact that, of the three selected, two are 

 by Allman must not be regarded as the result of a wish to unfavor- 

 ably criticize the work of this author. On the contrary the anatomi- 

 cal resemblances which he points out so clearly are worthy of the 

 most thoughtful attention, and although they are not homological and 

 do not indicate descent they are excellent illustrations of the inde- 



