420 GENERAL CONSIDERATIONS ON 



compound eye. It should also be noted that the ontogeny of 

 the Arthropod eye is as yet not satisfactorily explained. 



The respiratory organs of the Arthropoda must be dealt with 

 separately, according to the different phylogenetic series into which 

 they are to be divided. Since we derive the Arthropoda from forms 

 which live in water, it appears to us that the most primitive form of 

 respiratory organ must have been a tubular or leaf -like outgrowth 

 of the body-surface. Such a simple form of respiratory organ is 

 found in the gills met with as branchial tubes in the Annelida and 

 Crustacea. These branchial structures appear, as a rule, as append- 

 ages of the extremities. The gills of Limulus are also leaf-like 

 appendages of the abdominal limbs. From these we have to derive 

 the lung-sacs of the Arachnida (Scorpiones, Araneae), a fact indicated 

 by the method of development of these latter. In the transformation 

 of gills into lungs we recognise an adaptation to life on land. When 

 this adaptation goes further, it leads to the development of .unbranched 

 tracheal tufts (Araneae) which finally ramify in a dendriform manner 

 and develop a spiral filament (Pseudoscorpiones, Solifugae). In this 

 way is attained the same type of tracheal system as is produced in 

 different manner in other groups of Arthropods otherwise very far 

 removed from the above, viz., Peripatus, the Myriopoda, and the 

 Insecta. In the forms which were the starting-point of this last 

 series, the tracheae appeared as depressions of the body-surface, which 

 at first were irregularly distributed over the body (Peripafus), but 

 later attained to definite segmental arrangement. The tracheae in 

 the Myriopoda and the Insecta arose as such segmentally-arranged 

 depressions. The branches of the tracheal system are formed by the 

 splitting and branching of the original invaginations. In the Insecta 

 these tracheal rudiments appear very early, in the Myriopoda, on the 

 contrary, much later, and, as Peripatus in this way resembles the 

 Myriopoda, this late appearance of the tracheal rudiments has been 

 regarded as an indication of their having been recently accpiiired. 

 The similarity in structure between the tracheae of the Arachnida 

 and those of the Myriopoda and Insecta is remarkable. The presence 

 of the spiral filament in these two forms of tracheae, which must be 

 regarded as having arisen independently in the two groups, is specially 

 striking, but this feature loses its value as an indication of a common 

 origin when it is seen that such a spiral thread also occurs in other 

 tubes lined by a chitinous intima, e.g., the efferent ducts of glands 

 (salivary and spinning glands of the Insecta) and the vas deferens of 

 the Cijtheridae, p. 335). 



