COMPARATIVE MORPHOLOGY. 229 



ratus for the circulation and the aeration of the blood ; 

 a nervous system with sense-organs ; muscles and motor 

 mechanisms ; reproductive organs. Regarded as pieces 

 of ph3^siological 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 afiinit}' ; and that there is no affinity between the 



