COMPARATIVE MORPHOLOGY. 229 
ratus for the circulation and the aération of the blood; 
@ nervous system with sense-organs; muscles and motor 
mechanisms; reproductive organs. Regarded as pieces 
of physiological apparatus, there is a striking similarity 
between all three. But, as has already been hinted in 
the preceding chapter, if we look at them from a purely 
morphological point of view, the differences between the 
crayfish, the perch, and the pond-snail, appear at first 
sight so great, that it may be difficult to imagine that the 
plan of structure of the first can have any relation to 
that of either of the last two. On the other hand, if the 
crayfish is compared with the water-beetle, notwithstand- 
ing wide differences, many points of similarity between 
the two will manifest themselves; while, if a small 
lobster is set side by side with a crayfish, an unpractised 
observer, though he will readily see that the two animals 
are somewhat different, may be a long time in making 
out the exact nature of the differences. 
Thus there are degrees of likeness and unlikeness 
among animals, in respect of their outward form and 
internal structure, or, in other words, in their morpho- 
logy. The lobster is very like a crayfish, the beetle is 
remotely like one; the pond-snail and the perch are 
extremely unlike crayfishes. Facts of this kind are com- 
monly expressed in the language of zoologists, by saying 
that the lobster and the crayfish are closely allied 
forms; that the beetle and the crayfish present a re- 
mote affinity; and that there is no affinity between the 
