A STUDY IN MORPHOLOGY. 125 



VII. SERIAL HOMOLOGY AND BILATERAL SYMMETRY IN THE CRUSTACEA. 



The Phyllopods and the highest Brachyura are connected with each other by a 

 tolerably complete series of intermediate forms, and as we pass this series in review 

 we cannot fail to notice that, as has been so frequently pointed ont by morphologists, 

 each successively higher form is a little in advance of the one next below it in the 

 degree to which the functions and structure of the somites and appendages are 

 subordinated to the individuality of the organism as a whole. 



In the lower forms the body is made up of a series of nearly similar somites, and 

 the appendages, with the exception of those at the anterior end of the body, are 

 essentially alike in structure and their functions are indentical throughout the series. 

 The greater part of the body of such a Crustacean as Artemia consists of a series of 

 similar somites, and in Aptts we find more than sixty pairs of limbs which agree with 

 each other so perfectly in function as well as in structure that any one of them might 

 be substituted for any other without involving any essential change in the structure 

 of the animal as a whole. 



At the other end of the series we have Crabs with the primitive distinctness of the 

 somites so obscured by the centralised individuality of the whole organism that it 

 cannot be traced at all without careful study and comparison of various stages in the 

 life of a number of forms. 



Comparing the various appendages of a Crab with each other we find that their 

 functions are not at all alike. The mandibles are nothing but masticating organs, and 

 the power which they once had, and which they still retain in the Nauplius of Lucifer 

 to aid in locomotion, has entirely disappeared. 



Other appendages have become organs for procuring food, or weapons of offence or 

 defence ; others have become walking legs ; others long oars or paddles ; others again 

 have lost all limb-like functions, and are changed into accessory reproductive organs ; 

 whilst others again have entirely disappeared. 



In accordance with this specialisation of each appendage to a particular function, a 

 corresponding structural change has been brought about, and it is only after careful 

 study of the younger stages that we perceive the mandibles, maxilla?, foot-jaws, 

 walking and swimming legs, and copulatory organs of an adult Crab to be as strictly 

 homologous with each other as are the unspecialised appendages of Apus. 



The integration of the somites into a centralised whole has been accompanied by a 

 differentiation of each appendage from the others, and a specialisation to a restricted 

 function. 



An adult Crab resembles and differs from one of the higher Macroura in about the 

 same way that it resembles and differs from its own Meyalops larva, and the transition 

 from the larval form to the adult form is accompanied, like the transition from an 

 adult low Crustacean to a high one, by increased dependence of the various parts on 

 each other, by the increased prominence of the general individuality over the indivi- 



