THE ARRANGEMENT OF THE SEGMENTAL 



MUSCLES IN THE GEOPHILID^E, AND ITS 



BEARING UPON THE DOUBLE NATURE 



OF THE SEGMENT IN THE HEXAPODA 



AND CHILOPODA* 



The arrangement of the dorsal lateral longitudinal 

 muscles in the Geophilid^e corresponds to the division of 

 the segment into an anterior and posterior somite. This, 

 considered in connection with the presence of homologous 

 areas in Scolopendrella, Campodea, Japyx, Forficula, etc., 

 together with other evidence, notably the development 

 of the pterygodum (tegula) and wing of the mesothorax 

 in Lepidoptera, the double cross commissures in the em- 

 bryonic stages of Hexapoda and Chilopoda (as well as 

 Crustacea and Arachnida), the two pairs of metathoracic 

 tracheal openings in Japyx, etc., presents a strong case for 

 regarding the segment in the Hexapoda and Chilopoda as 

 composed of two somites, for which the terms protosom^ite 

 and deutosomite are proposed. 



The 'microthorax' to which Verhoeflf has recently 

 called attention as a fourth thoracic segment anterior to 

 the prothorax (Dermaptera) can not be homologized, as 

 he suggests, with the segment bearing the poison claws in 

 Chilopoda, inasmuch as this segment is composed of a 

 protosomite and deutosomite, the former being homol- 

 ogous with the microthorax (see Geophilidae) . Further- 

 more, a protosomite homodynamous with the 'micro- 

 thorax' is present in the Dermaptera on the mesothoracic 

 and metathoracic, as well as on the abdominal, segments. 

 Consequently there is evidence for considering that not 

 only is the thorax in Hexapoda composed of six somites, 

 but that each typical segment in the Hexapoda and Chil- 

 opoda (Crustacea and Arachnida?) is composed of two 

 coalesced somites. 



*Reprinted from Science, N. S., vol. 17 (1903), pp. 485-486. 



