524 INVERTEBRATE MORPHOLOOT. 



markable instance of convergent evolution. It is to be noticed that the 

 most primitive Insects, the forms through which affinities to the Crustacea 

 if they exist must be traced, are as a rule provided only with simple eyes, 

 a condition repeated in the eyes of Insect larvae — a fact which indicates that 

 the compound eyes are structures which were not characteristic of the 

 primitive Insects, but have developed within the limits of the group and 

 can therefore have no phyletic connection with the compound eyes of the 

 Crustacea. Adding to this fact the independently-developed tendency to 

 form compound eyes seen in certain Annelida and Pelecypod MoUusks, it 

 seems probable that notwithstanding their remarkable structural similari- 

 ties the compound eyes of Crustacea and Insects have been independently 

 acquired. Instead, therefore, of uniting the three groups together as a type 

 Arthropoda equivalent to the other types, it seems preferable to separate 

 them as distinct, just as is done with the Annelida and Mollusca, and the 

 Annelida and Prosopygia. 



Starting, then, with the supposition that Peripatus has descended from 

 Annelid ancestors and represents the ancestors of the Myriapoda, the rela- 

 tionships of the various orders of this class and of the Insects remains to 

 be traced. Unfortunately a large gap exists between Peripatus and any 

 recent Myriapods, and it is possible that this class is a heterogeneous group ; 

 indeed by some recent authors it has been suggested that it should be 

 done away with as a class, the Ohilopods being united with the Insecta to 

 form one class, while the Diplopods (perhaps with the Pauropoda associated 

 with them) should form a second. There is no doubt but that Peripatus 

 possesses many tracheate peculiarities, but its affinities to the remaining 

 Tracheates are much more remote than those which exist between the vari- 

 ous groups of Myriapoda, or between any of these groups and the Insecta. 

 The character of the various appendages considered in relation with the 

 nervous system seems to afEord an admirable means of indicating the rela- 

 tionships of the various groups. The brain of Peripatus seems to be 

 formed by the fusion of three pairs of ganglia ; the most anterior and dor- 

 sal of these gives rise to the antennal nerve and the most posterior inner- 

 vates the mandibles, while upon the middle one, which is closely related to 

 the mandibular ganglion, the eye seems to be placed. It may be assumed 

 that the ganglia with which the eyes are associated represent the Annelid 

 supraoesophageal or cerebral ganglia and may therefore be termed the pro- 

 tocerebrum, while the antennary ganglia form the deutocerebrum, and the 

 mandibular the tritocerebrum. In the Myriapods and Insects the brain is 

 also composed of three parts to which the same names are applied, the 

 antennae being innervated from the deutocerebrum, while the tritocerebrum 

 lacks a corresponding appendage, though in certain Insects transient indi- 

 cations of a tritocerebral appendage have been seen. Bearing these facts 

 in mind, the ganglia and appendages of the various groups may thus be 

 tabulated, and to make the comparison complete the Crustacea are also in- 

 cluded. 



