FROM EMBRYOLOGICAL DATA. 27 



pupte and the different types of Crustacea, and that I have satisfied myself that it 

 will successively be found more and more intimate, and give finally to this classifi- 

 cation the character of a true genetic natural arrangement. 



With reference to the class of Insects, there are some particular points which 

 require especial study. It is well known that among air-breathing Articulata there 

 have been several classes distinguished under the name of Insects proper, Arach- 

 nidians, and Myriapoda, and there are several groups intermediate between the two 

 latter, which stand almost isolated, and were formerly united under the common 

 name of Aptera. The question now arises, how these are to be regarded with 

 reference to the true Insects, and whether they should form intermediate classes 

 between Crustacea and Insects, or be united with the Insects proper as one class. 



As far as the Arachnidans are concerned, I entertain no doubt that they cannot 

 be considered as a class by themselves, but must be decidedly united with the 

 Insects; for the peculiarities in their structure upon which the separation rests 

 hardly justify such a primordial distinction. The difference, as it has been given, is 

 chiefly derived from the number of legs, and from the respiratory apparatus ; there 

 being in the true Insects only three pairs of legs, while in Spiders there are four, and 

 the respiratory organs of Insects being true tracheae, and those of spiders lung-like 

 sacs. But this greater number of locomotive appendages indicates only a lower 

 degree of structure, rather than a classic difference. Again, the difference in the 

 structure of the respiratory organs is rather morphological than essential, as Dr. 

 K. Leuckardt has recently shown. So that the chief ground for a distinction of the 

 Spiders as a class is illusory. But there is a far more important difference in the 

 circumstance, that in Spiders head and chest are united ; and this corroborates the 

 inferiority which an additional pair of legs ascribes to them among Insects. Spiders, 

 indeed, should be considered as pupa-like Insects in a perfect state of development, 

 not undergoing a further growth, but assuming, in that stage of progress, their final 

 development as perfect animals. The greater resemblance of their jaws and anten- 

 nse to legs would also sustain this view. 



As for the Myriapoda, we can consider them as caterpillar-like Insects, in which 

 the respiratory organs, the masticatory organs, and the organs of locomotion, assume 

 their final growth, under a worm-like form, and the whole body presents true ento- 

 mological characters at a period of development when it still preserves the form of 

 a larva with many joints, with legs upon all the joints, and the head only distinct 

 from the other rings, but resembling Insects in their antennae, in their compound 

 eyes, in their articulated feet, in their stigmata, and in almost every detail of inter- 

 nal structure. This resemblance is even so close to the structure of some Coleoptera, 

 that one is almost tempted to view the Brachelytra as a connecting link between 

 this lower order of Insects and the Myriapoda. Fig. h of the foregoing wood-cut, 

 which represents a species of Polydesmus, will show this analogy between Myria- 

 poda and the larva of Insects, as well as with the Worms and lower Crustacea, and 

 Fig. g, which represents a Spider, Salticus scenicus, will show the corresponding 

 analogy between this group of Articulata and the pupa of Insects, as well as with the 

 higher Crustacea. In the highest class of Articulata, including Myriapoda, Arach- 



