1894,] The Classification of the Arthropoda. 229 
difficult to say whether the lack of other structures characteris- 
tic of Arthropods is due to primitive simplicity or to degener- 
ation. 
IiI—Pavropopa. 
The position of the Pauropoda is, as yet, very uncertain, as 
we are almost entirely ignorant of their internal structure. In 
the tendency towards a fusion of somites, in the lack of a sec- 
ond pair of maxille, and in the positions of the external paired 
openings of the genital ducts at the base of the second pair of 
ambulatory appendages and the non-motile spermatozoa they 
show undoubted affinities with the Diplopoda ; but the pecul- 
iar triramous antenne and especially the characters of the 
hexapod young, as figured by Lubbock (67) and Ryder (79) 
show important differences. The following table compares the 
somites of Pauropus and the Diplopod: 
DIPLopop. PAUROPOD. 
Appendage I Antenna Antenna 
oe II Mandible Mandible 
pad a | PS Lower Lip Lower Lip 
n IV Foot 1 Absent 
a Vv Absent Foot 1 
a VI Foot Foot 2 
~VE Foot 3 Foot 3 
IV—TARDIGRADA. 
Elongate metameric animals with four pairs of appendages, 
_ each terminating with two double hooks. Mouth and anus 
terminal, Malpighian tubes present, opening into the hind-gut. 
Nervous system consisting of a suprawsophageal brain and a 
chain of four ventral ganglia. No specialised circulatory or 
respiratory organs. No coxal glands or nephridia. Sexes 
Separate, gonad unpaired, emptying into hind-gut. 
Most frequently the Tardigrades are associated with the 
Arachnida, but this has doubtless been due to the possession of 
four pairs of functional legs in the two groups. These forms 
