133 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



case. Consequently the mouth-folds cannot represent the Crustacean 

 second autennse. My own views as to the homology of the mouth-folds, 

 already implied by my use of the term "gense," will presently be 

 supported. 



Hansen's recognition of the similarity between Campodea, Japyx, and 

 Collembola is sustained by embryology. In Campodea, Uzel ('98, p. 33) 

 describes and figures a " Chitiustrang . . ., welcher sich von der Yorder- 

 randmitte der zweiten Maxille um die Aussenseite des ersten ^[axille 

 und der Mandibel herum zu den auf deu lutercalarsegmente gelegenen 

 Hockei'u zieht." His Figures 38 and 79 show the Chitiustrang at a 

 rather advanced stage of development, corresponding with the condition 

 iu my Figui-e 17 ; unfortunately he gives neither its origin nor its earlier 

 development. The later development, as evidenced diagrammatically by 

 his Figures 80-84, agrees with that of Anurida in the gradual approxi- 

 mation of the lateral ridges, and especially iu the completion of the 

 buccal boundary by the same method of confluence. Uzel does not 

 attempt to explain the homology of the Chitiustrang. 



In Lepisma and Machilis the mouth-parts are ectognathous, as in 

 Orthoptera. In Lepisma there is no trace of a lateral mouth-fold, but 

 in Machilis I have found a distinct, flat, lateral lobe sheltering the base 

 of each mandible, and the lobe is probably homologous with the CoUem- 

 bolan mouth-fold. 



In Pterygota the geux, often not demarcated as distinct sclerites, 

 represent the lateral regions of the germ hand — as they do iu Campodea 

 and Collembola. In these Apterygota the same areas have simply in- 

 creased as folds, but the folds are none the less homologous with the 

 pleui'al regions of other insects, and in Collembola are reasonably to be 

 regarded as the pleural portions of the premandibular, mandibular, and 

 both maxillary segments. In many Pterygote insects, especially iu 

 Orthoptera, the genre overlap the bases of the jaws; for example, in 

 Caloptenus, in which the gena is produced as a small but distinct flat 

 fold over the base of the mandible. 



Little is known about the development of the sides of the liead in 

 Myriopoda, but in Peripatus it is interesting to find distinct lateral 

 mouth-folds (Sedgwick, '88, Plate II., Figure 36) quite analogous, to say 

 the least, with those of Collemhohi. 



Concerning the completion of the skull, little remains to be said. At 

 Stage 7 a constriction encircling the blastoderm separates the head from 

 the thr.rax. The head is typically a hollow cylinder, or cone, and so is 

 the body. The body cylinder consists of a definite number of successive 



