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 the 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 say 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 convenient 
way 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 ? 
