126 MR. W. K. BROOKS ON LUCIFER: 



dualities of the somites or metameres, and by the increased structural and functional 

 specialisation and differentiation of each appendage as compared with the others. 



This series of changes is so well exemplified by the study of adult and larval 

 Crustacea ; it is so remarkable and interesting ; so very conspicuous and unquestion- 

 able, that it has long attracted the attention and called forth the speculation of 

 morphologists. It is natural to suppose that the process of change which is open to 

 our observation through study and comparison of living Crustacea, is a continuation 

 of a, similar process which went on in the remote past. There seems then at first 

 sight to be reason for believing that, if we could go far enough back, we should find 

 the individuality of the whole organism gradually disappearing and giving place to 

 the separate individualities of the component somites ; that we should find the 

 specialisation of the appendages gradually disappearing, until we should at last find, 

 as the remote ancestor of the Crustacea, a series or community of independent 

 organisms, each one essentially like the others, and able to provide for its own wants 

 and to lead an independent existence when accidentally or naturally detached. 



This view has been advocated at length by H^ECKEL (' Generelle Morphologie,' 

 1866) and by SPENCER ('Principles of Biology,' vol. xi., 1867), and used by both 

 these writers as an explanation of the origin of all segmented or compound animals 

 and plants. It has been accepted, with more or less qualification, by many other 

 writers, although HUXLEY (' Oceanic Hydrozoa : ) and METSCHNICKOFF (Zeit. f. Wiss. 

 Zool., xxiv.) have pointed out that, even in the Siphonophorte, where the individualities 

 of the units in the compound are extremely well marked, the view that the organism 

 has been evolved by the gradual integration and specialisation of originally inde- 

 pendent Zooids is attended with serious difficulties. 



So far as we can see there is no reason why the Crustacea might not have originated 

 in this way, by the gradual integration and differentiation of a community of inde- 

 pendent metameres, but the evidence which is attainable seems to directly oppose the 

 belief that this has actually happened. We are able to trace the higher Decapods 

 back, very satisfactorily, to a Phyllopod-like ancestor with a long series of undif- 

 ferentiated somites and appendages, but even here the somites are simply parts of 

 the body, and they furnish no more evidence than those of a Crab to show that they 

 ever were the independent organisms of a community. 



When we attempt to go still further back we find that the facts of embryology, if 

 they show any thing whatever about the phyllogeny of the Crustacea, lead us back to a 

 Nauplius with three inter-dependent somites and three pairs of specialised appendages, 

 rather than to a form with a great number of unspecialised somites and similar 

 appendages. 



Turning now to a somewhat different aspect of the subject, we notice that, if we 

 confine ourselves to structure, and leave out of sight the question of origin, there is 

 the closest similarity between serial homology and the homology between the corre- 



