ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 949 



j3. Myriopoda. 



New Species of Myriopoda.* — Mr. J. M'Neill gives descriptions of 

 twelve new species of Myriopods, chiefly from Indiana. Hexaglena is the 

 name applied to a new genus, in which there are six eyes, arranged in two 

 divergent lines, close to the bases of the antennae ; the head is conical and 

 minute, and there are spiracles in one row on each side of the body. The 

 new genus differs from its nearest allies Octoglena and Petaserpes, in that 

 the former of them has eight eyes, and the dorsal aspect of its head exposed, 

 and the latter has only two eyes, while its spiracles are arranged in two 

 rows. H. cryptocephala is a new species. The other new forms are 

 Polydesmus castaneus ; Trichopetalum hollmani, which is allied to T. glomera- 

 tum ; Lisiopetaliim endasym ; lulus inultiannulatus, v/hich is 16.5 mm. long, 

 and is the largest species of the genus yet described from North America ; 

 Geophilus hrunneus, G. indianee, and G. varians ; Mecistocephalus strigosus, 

 and M.foveatus; and Scolopocryptops nigridius, which, in general appearance 

 and habits resembles Litliohius. 



S. Araclinida. 



Phylogeny of Araclmida.l — -In discussing the systems of organs in 

 the Arachnida, Herr B. Weissenborn rightly commences with the nervous 

 system, as questions of homologies between the appendages can only be 

 answered by reference to the innervation. The nervous system of Arach- 

 nids is distinguished from that of most Arthropods by the absence of 

 antennary nerves, but there are many points of agreement which show us 

 that the system in all Arthropods exhibits a more or less well-marked 

 segmental development and distribution ; they all agree in having the parts 

 derived from epiblastic thickenings, but in the Crustacea the rudiments are 

 continuous, while in Arachnids and Myriopods, the rudiments of the central 

 portion are distinct from those of the ventral medulla, and in insects they 

 are only loosely connected. So far then the Arachnida agree with the 

 Insecta and Myriopoda. With regard to histological structure and composi- 

 tion they all agree. The Tardigrada and the Pycnogonida present con- 

 siderable variations from what is normal among the Arachnida. 



The dermal skeleton of Arachnids, like that of other Arthropods, is a 

 product of the integument, and differs considerably both in its qualitative 

 and its quantitative development ; the most various relations obtain with 

 regard to external jointing. Here again the Tardigrada and the Pycnogonida 

 are the most abnormal. In the former, and in the Acarina, the homonomy 

 of the body segments, and the union of the anal and genital orifices are 

 sharp marks of distinction, and coupled with other causes, seem to show 

 that the Tardigrada are the offshoots of a branch of the articulate phylum, 

 which separated off much earlier than that of the Arachnida, and perhaps 

 even than the Arthropoda. 



The appendages are next considered ; the diminution in the number 

 possessed by the Linguatulida and their small size may be explained by 

 the cestode-like mode of life of these forms. Here, as in the dermal 

 skeleton and the nervous system, the Scorpionida and Solpugida are groups 

 which exhibit' a primitive character in many of their characters, and they 

 must therefore be regarded as standing near an older stem-group. The 

 Pycnogonida are again aberrant, while the Tardigrada give just the same 

 kind of evidence with their appendages as with their dermal skeleton. 



The respiratory organs are not to be regarded as modified gills, but 



* Proc. U.S. Nat. Museum, 1887, pp. 323-34 (1 pi.). 



t Jenaisch. Zeitschr. f. Naturwiss., xx. (1887) pp. 33-119. 



