49 6 THE ANIMAL KINGDOM. 



possess the power of casting off limbs ; all reproduce them when injured. 

 A similar reproduction has been observed in some Insecta, Myriapoda, and 

 Spiders. Mimicry of colour and shape is frequent, and in some Crustacea 

 the expansion and contraction of pigment cells (chromatophores) may 

 assimilate the tint of the organism to its surroundings. The life-history of 

 a Crustacean is primarily, of an Insect secondarily, connected with a meta- 

 morphosis. In the higher Crustacea the metamorphosis becomes restricted 

 or lost, in the higher Insecta more pronounced. There are but slight indi- 

 cations of similar changes in the other classes. Degraded forms, with 

 regressive metamorphosis, are found e.g. in many parasitic Copepoda among 

 Crustacea, Linguatulina among Arachnida. 



There are five classes of Arthropoda usually recognised, the Insecta s. 

 Hexapoda, the Myriapoda^ the Protracheata, the Arachnida, and Crustacea, 

 to which must be certainly added, as an independent group, the Pycnogo- 

 nidae s. Pantopoda. The Insecta may perhaps have some relationship with 

 the Myriapoda through the genus Scolopendrella. The class Protracheata, 

 represented by the genus Peripatus, stands quite alone. But the three 

 classes first-named agree together, and differ from the two last in having 

 praeoral cephalic processes or antennae. The praeorally placed appendages 

 of Arachnida and Crustacea appear to be postoral limbs, which have 

 acquired a secondary praeoral position. It has been proposed in con- 

 sequence to unite them under the common term Acerata. The majority of 

 Arachnida are tracheate, and by some authorities the Insecta, Myriapoda, 

 Protracheata, and Arachnida are therefore grouped together as Tracheata. 

 But it does not appear certain that the tracheae of the four classes in 

 question are homologous structures, and the Arachnida, as given below, 

 include certain branchiate forms, viz. Limulus, and the extinct Eurypterina 

 and Trilobita. The young Limulus has a singular resemblance to the 

 extinct Prestwichia. Many Trilobita have a similar larval form, and it is 

 possible that the Scorpions and Eurypterina may be traced back to the 

 same type. There are however certain sub-groups the position of which is 

 exceedingly doubtful. These are the Linguatulina, the Tardigrada, and 

 Acarina. They are generally classed among Arachnida, and are retained 

 in that position here. It is possible, however, that they may have to be 

 separated, and this is especially the case with the Acarina, which are placed 

 apart by Haller in a Class, Acaroidea. 



There are many interesting points relative to the connection of the classes of 

 Arthropoda and certain of their sub-groups which cannot be discussed in a short 

 compass. Reference must be made to the authorities cited below. 



Haller gives the following characters for the class Acaroidea (Z. A. iv. 1881, p. 

 386) : " Wingless Arthropoda, usually of small size, respiring partly through the 

 integument, partly by tracheae. Cephalothorax and abdomen as a rule indis- 



