CONSIDERATIONS. 9 



begin to tend towards the head, following a degradation in 

 order of rank, to transform themselves into masticatory organs. 



On the other hand, the posterior feet, which are maintained 

 as locomotive organs amongst the Myriapoda, instead of dis- 

 appearing amongst the Crustacea, as in the Insecta, are trans- 

 formed successively into organs of respiration, commencing 

 with the last pairs : and, advancing more and more towards the 

 anterior pairs, we find that, in the Stomapoda, there exist but 

 three pairs of legs capable of walking, whilst the others are 

 changed on the one hand into the organs of the mouth ; on the 

 other, into organs of respiration. Amongst the Branchiopoda 

 which terminate the Crustacea, all the feet, with the exception 

 of a single pair, become branchiferous, and the only one which 

 has not undergone this transformation is of no service in 

 walking, but only in swimming. Lastly, in the Cirrhopoda, 

 no pair of feet serves for progression, but simply for pre- 

 hension, and it is probable they fulfil also the functions of 

 branchiae. 



The feet in Gammarus present us with a remarkable instance 

 of this change of function. The anterior are transformed into 

 jaws; the following become chelate legs {jneds macho'tres), 

 and serve for prehension. Of the middle pairs, the anterior 

 remain ambulatory, the posterior serve for swimming, whilst 

 the hinder pairs are transformed into branchiae. 



Fourth Law. — When the function demands a greater 

 complication in the composition of the organ which has to 

 perform it, this last acquires it successively, by parts, which 

 are added at first under the form of rudiments. 



These rudiments are further developed in other species, or 

 in other pairs of the same organs, so that these have often 

 undergone a very considerable change of form, before it 

 becomes probable that they have changed function : this is 

 particularly remarkable in the feet of Crustacea, which, from 

 ambulatory organs, become branchiferous. 



Fifth Law. — The same functions may be exercised by 

 very different systems of organs {appareils). 



In all circumstances where Nature could not preserve an 

 organ, or system of organs ; whether that it could not accord 

 itself with the rest of the organization of the animal, or that, 

 after the degradation it had undergone, it had disappeared, 

 whilst it was necessary that its functions should be preserved ; 



NO. I. VOL. I. C 



