i 



XIX.] MORPHOLOGY. 235 



pilosis resemble articulated animals of a lower organization : 

 caterpillars resemble centipedes, and maggots resemble 

 worms. The Batrachians (frogs, toads, and newts), which Batra- 

 are the lowest air-breathing vertebrates, in their larva or ^ ^^°^' 

 tadpole state resemble some of the most lowly organized 

 fishes, which are below the Batrachians in grade of 

 organization. The crabs are the highest of the Crustacea, 

 and their larvae are unmistakeably crustacean, but crusta- 

 cean of a lower grade. But the law that metamorphosis is 

 accompanied by advance in grade of organization is only a , ^ 

 general law, not a universal one. Among the Crustacea tacea. 

 there are some remarkable exceptions to it ; the largest of 

 which, though not the only one, is that of the great order qjj.j.Jj- 

 of Cirrhipedes, or Barnacles, which are decidedly crusta- pedes, 

 cean, but in their mature state are metamorphosed into 

 fixed animals covered with shells, and having so great a 

 resemblance to the Mollusca that they were classed as 

 such until their larval forms became known. This must 

 be regarded as a case of retrograde metamorphosis, or Eetrograde 

 metamorphosis to a lower type ; for the larva, which has "^tamor- 

 powers of sight and motion, is certainly a higher being 

 than the mature animal, which has neither. Among the 

 Echinoderms (star-fishes and sea-urchins) there are some Echino- 

 very extraordinary larval forms, which have no resem- 

 blance whatever to the perfect form ; but perhaps this 

 exception to the usual law of metamorphosis is rather 

 apparent than real. I shall have to speak of them in the 

 chapter on Embiyology. 



We have seen that there are three cases of unlikeness of 

 form within the limits of the same species. These are — 



1. Between the larval and the mature forms ; 



2. Between the sexes ; and 



3. Between the forms that alternately produce each 



other by metagenesis. Under this last head, 

 from a physiological point of view, comes, as we 

 have seen, the relation between the leaf-bearing 

 and the flower-bearing axes of plants. 

 In all these three cases, as a general rule, the differences ^"^^^"1" 

 are not fundamental : what may be called the ground-plan betweer. 



