204- THE MORPHOLOGY OF THE COMMON CRAYFISH. 



with its whorls of leaves, as a crayfish's head has to its 

 abdomen, or a dog's skull to its thorax. 



It may be objected, however, that the morphological 

 generalisations which have now been reached, are to a 

 considerable extent of a speculative character ; and that, in 

 the case of our crayfish, the facts warrant no more than 

 the assertion that the structure of that animal may be 

 consistently interpreted, on the supposition that tlie body 

 is made up of homologous somites and appendages, and 

 that the tissues are the result of the modification of 

 homologous histological elements or cells ; and the ob- 

 jection is perfectly valid. 



There can be no doubt that blood corpuscles, liver 

 cells, and ova are all nucleated cells ; nor any that the 

 third, fourth, and fifth somites of the abdomen are con- 

 structed upon the same plan ; for these propositions are 

 mere statements of the anatomical facts. But when, from 

 the presence of nuclei in connective tissue and muscles, 

 we conclude that these tissues are composed of modified 

 cells ; or when we sa}^ that the ambulatory limbs of the 

 thorax are of the same type as the abdominal limbs, the 

 exopodite being suppressed, the statement, as the evi- 

 dence stands at present, is no more than a coavenient 

 waj^ of interpreting the facts. The question remains, 

 has the muscle actually been formed out of nucleated 

 cells ? Has the ambulatory limb ever possessed an 

 exopodite, and lost it ? 



