142 BULLETIX: MUSEUM OF COMPARATIVE ZOOLOGY. 



\Yheeler ('89, p. 568) finds that five pairs of ectodermal iuvagiuations 

 form the tentorium of the larval head of Doryphora. "These iuvagiua- 

 tions grow inwards as slender tubes, wliich anastomose in some places. 

 Their lumina are ultimately filled with chitiu." "Wheeler offers his 

 observations in support of Palmeu's theory, but they are not at all 

 inharmonious with the scanty observations I have made upon Anurida. 



Heymous ('95'', pp. 50-51), describing Forficula, agrees with ^Yheeler. 

 except that he finds only two pairs of fundaments for the tentorium, and 

 says (p. 51): " Ich habe mich indesseu davon uberzeugt, dass auch bei 

 Gryllus und Periplaneta die zahl der Tentoriumanlageu keine grossere 

 ist, sondern, wie Heider ('89) dies bei Hydrophilus beschrieb, und ich es 

 bei Forficula fand, nur vier betragt. Der oben geschilderte Entwicke- 

 lungsmodus des Tentoriums dlirfte daher wohl als der typische anzuseheu 

 sein." 



In Anurida I was unable to find any distinct ectodermal invaginations 

 which might form the arms of the tentorium, but am not prepared to say 

 that none exist, because the subject is one of great difficulty. The arms 

 must be studied in oblique sections, and it is almost impossible to dis- 

 tinguish them from fundaments of muscles until they are nearly com- 

 pleted. The finished tentorium of Collembola, however, is undoubtedly 

 homologous with that of Thysanura, and almost as clearly with the ten- 

 toi'ium of Orthoptera. 



Segmentation of the Head. 



The elucidation of the primitive segments in Arthropods is a most 

 interesting and difficult morphological problem. The rule of Saviguy, 

 — emphasized by Huxley and others, — that Arthropods consist funda- 

 mentally of successive rings, each of which may bear but one pair of 

 primary appendages, although now undoubted, has never been thoroughly 

 substantiated when applied to the Hexapod head. After years of argu- 

 ment, morphologists still disagree as to the number of somites composing 

 the highh' difFerentiated heads of insects. Kolbe ('90, p. 135) recognizes 

 five, as follows : — 



1. Ursegment : FUhler, Augen, Oberlippe ; 



2. Ursegment : Oberkiefer oder Mandibeln (1. Kiefernpaar) ; 



3. Ursegment: Unterkiefer oder Maxillen (2. Kiefernpaar); 



4. Ursegment : Zunge oder Innenlippe (3. Kiefernpaar, verwachscn) ; 



5. Ursegment : Unterlippe (4. Kiefernpaar, verwachsen). 



Sharp ('95, p. 87) says, " Morphologists are not yet agreed as to tlieir 



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