224 THE ANATOMY OF INVERTEBRATED ANIMALS. 



genetically, at another time undergo fecundation. Multipli- 

 cation by fission or external gemmation is not known to take 

 place in any Arthropod. Hermaphrodism occurs as a rule in 

 some few Arthropods (e. g., the Cirripedia and Tardigrada), 

 and as an abnormal "sport" in sundry Crustacea and in 

 many Insecta. 



In absolute number of species, the Arthropoda far ex- 

 ceed all the rest of the animal kingdom put together. Thus 

 Gerstaecker, 1 while allowing 50,000 species for the latter, 

 estimates the number of species of Arthropoda as rather 

 above than below 200,000 ; by far the larger proportion of 

 these, probably more than 150,000, being Insects. 



The Arthropoda are commonly divided into the Crustacea, 

 the Arachnida, the Myriapoda, and the Insecta ; and though 

 it is impracticable to give a definition which shall absolutely 

 separate the first two groups, it is perhaps not worth while 

 to disturb an arrangement which has much practical con- 

 venience. But, for purely morphological purposes, it may be 

 instructive to regard them from another point of view. 



The Arthropoda may, in fact, be divided into two series. 

 One of these consists almost wholly of air-breathing forms, 

 which, if they possess special respiratory organs, have either 

 pulmonary sacs or tracheae, or both combined ; while the 

 other includes a corresponding predominance of water-breath- 

 ing animals, which, if they possess respiratory organs, have 

 branchia?. The latter series contains the Crustacea; the 

 former comprises the Arachnida, Myriapoda, and Insecta. 



In the course of the development of the higher Arthro- 

 poda, there is a stage in which the body begins* to be seg- 

 mented, but the appendages are not developed. This is 

 followed by a stage in which appendages make their appear- 

 ance, but the antennary and manducatory appendages (gna- 

 thites) are like the other limbs : and, finally, there is a stage 

 in which the gnathites are completely converted into jaws. 

 Now, among the water-breathing Arthropoda, no trace ^ of 

 limbs has yet been certainly discovered among the Trilo- 

 ba a ; in the Merostomata (Eurypterida and Xiphosura) 

 the gnathites are completely pediform ; while, in the Ento- 

 mostraca and Malacostraca, more or fewer of the gnathites 

 are so modified as to subserve manducation and no other 

 function. 



1 Bronn\s "Klassen und Ordnungen des Thierreichs," vol. v., p. 273. 1868. 



