19* PAL/EONTOLOGY 



living forms with so similar a general structure of 

 skeleton that we can safely refer them to the same 

 phylum. This is the great phylum Arthropoda, which in- 

 cludes the Insects, the Myriapods (centipedes and milli- 

 pedes), the Arachnids (scorpions and spiders), the 

 Crustacea (lobsters and crabs), and one or two minor 

 and unfamiliar classes. In all these, the body is meta- 

 merically segmented (divided into a longitudinal series of 

 more or less similar somites), and encased in an exo- 

 skeleton of chitin which may be hardened by a deposit of 

 calcium phosphate ; a number of the somites at the 

 anterior end are more or less completely united into a 

 head ; and on the ventral surface is a paired series of 

 jointed limbs (usually one pair to each segment) which 

 fulfil the functions of locomotion (walking or swimming), 

 the seizing of food, and in aquatic forms respiration. 



Shell-growth in the Arthropoda is totally unlike that of 

 Brachiopoda and Mollusca. The exo-skeleton is not 

 secreted by a mantle but by the whole surface of a com- 

 plex jointed body. It is a continuous cuticle, thickened 

 and hardened to form a number of rigid pieces (sclerites) 

 but remaining thin between, where flexibility is neces- 

 sary. Marginal growth is therefore impossible. The 

 growing animal at intervals bursts and casts off its 

 whole exo-skeleton, and presently secretes a new and 

 larger one : this is the process of ecdysis or moulting. The 

 only way in which the ontogeny of a fossil arthropod can 

 be determined is by the finding of a series of cast skins 

 (or of prematurely dead animals) : this has been success- 

 fully done for a number of species of trilobites. 



