35 



CHAPTER III. 



TRACES OF UNITY IN THE APPENDICULAR 

 ORGANS OF INVERTEBRATE ANIMALS. 



THE parts of invertebrate animals which may be rightly 

 regarded as limbs are very many and very varied, and 

 not a little patience is necessary in order to arrive at 

 any clear conclusions respecting them. 



Taking a common prawn (Pal&mon squilla) and com- 

 paring it with other crustaceans, and with other inverte- 

 brate creatures generally, it is easy to see that these 

 parts are, not only the five pairs of legs which entitle the 

 creature to the name of decapod, but also the false ab- 

 dominal legs, the foot-jaws, the mandibles and maxillae, 

 the antennae, the eyes, and certain other appendicular 

 organs as well. 



I. In the prawn the ten true feet, all of them sub- 

 stantially alike, are arranged bilaterally in five pairs. 

 Each " foot " is articulated or divided by joints into six 

 inter-articular or nodal parts, of which the names, reck- 

 oning from above downwards, are coxa, troehanter, femur, 

 tibia, tarsus, and metatarsus. The interarticular parts 

 are composed of a hard external crust and an internal 

 cavity in which are the muscles and other soft parts. 

 In the three hinder pairs the metatarsus is prolonged 

 into a sharp stiff claw : in the two first pairs the foot 

 terminates in a forceps or chela, the tarsus being elon- 



D 2 



