30 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



position and segmentation corresponds to the mesoblast of Arachnids and Peripatus, 

 has usurped the functions of the true endoderm. 



In Aphides from the studies of Witlaczil it appears that the intestine is formed 

 exclusively from the proctodaeal and stomadseal invaginations. 



In order to separate the different divisions of the arthropod phylum anatomical 

 characters as well as embryological phases must be considered. 



The posession of a single well developed pair of antennse, of tracheal invagina- 

 tions and of embryonic membranes, and the existence of a hexapod stage in their 

 development afford sufficient ground for regarding myriapods as lowly organized or 

 degenerate insects. Peripatus would perhaps come under the same category through 

 the embryonic membranes of Peripatus do not appear to correspond fully to those of 

 insects. 



Arachnids, from the absence of antennse and the histological structure of the 

 abdominal appendages, and from other characters, anatomical and histological, must 

 certainly be included, with Limulus, in a distinct group of- arthropods. The small 

 seventh pair of thoracic appendages of Limulus is perhaps an interpolated append- 

 age, or perhaps a corresponding appendage may be discovered in the development 

 of spiders and scorpions. 



Arachnids, probably, never possessed antennse, since all their appendages, like 

 those of Limulus, are at one period post oral, and are not innervated by the supra- 

 oesophageal ganglion. Trilobites, possibly the ancestrial form of Limulus, from 

 evidence afforded by the Cincinnati specimen, probably possessed no antennse. 

 If antennse were ever present in the group we would expect to find them in these 

 old forms. 



The antennse of insects from their iunervation correspond to the first pair of 

 crustacean antennse. The bilobed upper lip of insects is innervated from the second 

 division of the supra-cesophageal ganglion which forms part of the circum-cesopha- 

 geal commissure. In the nauplius stage the second pair of crustacean antennse is 

 innervated from the circum-cesophageal commissure. From their similar innerva- 

 tion a comparison may then be fairly drawn between the paired upper lip of insects 

 and the second pair of crustacean antennas. 



