( 515 ) 



XXXVIII. — On the Morpliological Relationships of the Molluscoida and Coelenterata, 

 and of their leading Members, inter se. Bj John Denis Macdonald, R.N., 

 F.R.S , Surgeon of H.M.S. " Icarus." 



(Read 21st December 18G4.) 



Few departments of zoology have recently suffered more remarkable changes, 

 both in classification and accepted views of structure, than the Polypi or Coelen- 

 terata, and their immediate allies in the ascending scale, the Molluscoida,— greatly 

 depending upon the more extended study of those animals of late years. We 

 have been thus enabled to discover natural affinities which prima facie evi- 

 dence would scarcely ever have indicated, as well as intrinsic differences which 

 the same kind of evidence has hitherto been incapable of revealing to the mind. 



Leading from the Protozoa to the Mollusca proper, the Coelenterata and Mol- 

 luscoida constitute an unbroken series of animals forming a considerable section 

 of invertebrata, distinguished from the Protozoa by the development of true ova, 

 and from the Mollusca by the property of gemmation developing compound ex- 

 amples of the principal types. Furthermore, the motion of the blood, or its equi- 

 valent, is effected either by ciliary action or by a propulsive organ ; but when the 

 latter occurs it is unfurnished with valves, so that the course of the circulation 

 may be reversible in the same canals. 



The study of the different stages of development of a certain organ in the 

 same animal comes within the pale of ordinary physiology : but when M^e pry 

 into the progressive advance of any organ or function, taken in the abstract, we 

 enter upon a more comprehensive branch of science, which not only embraces the 

 common physiology of each particular animal, but its combined import in all. 

 On comparing the relative parts of two distinct animals, one unaccustomed to a 

 study of this kind could scarcely doubt that the mouth of one was anything more 

 or less than the exact equivalent of the mouth of the other ; but it may be clearly 

 shown that mouths acting as such, so far as simple function is concerned, may 

 nevertheless exhibit a remarkable difference, homologically speaking, in animals 

 constructed on different types. 



It was formerly believed that the branchial and cloacal orifices of the Asci- 

 dian were homologous with those of the siphonal tubes of Lamellibranchiata ; 

 but this very natural error has been pointed out by Professor Huxley, and it can- 

 not be doubted that the orifice of ingress is in reality oral in one, while it is simply 

 pallial in the other. In the Ascidian, moreover, the inner or commonly recog- 

 nised mouth is but the oesophageal opening, though it is critically answerable to 



VOL. XXIII. PART II. 7 A 



