142 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Wheeler (’89, p. 568) finds that five pairs of ectodermal invaginations 
form the tentorium of the larval head of Doryphora. ‘These invagina- 
tions grow inwards as slender tubes, which anastomose in some places. 
Their lumina are ultimately filled with chitin.” Wheeler offers his 
observations in support of Palmén’s theory, but they are not at all 
inharmonious with the scanty observations I have made upon Anurida. 
Heymons (’95", pp. 50-51), describing Forficula, agrees with Wheeler, 
except that he finds only two pairs of fundaments for the tentorium, and 
says (p.51): ‘Ich habe mich indessen davon tiberzeugt, dass auch bei 
Gryllus und Periplaneta die zahl der Tentoriumanlagen keine gréssere 
ist, sondern, wie Heider (89) dies bei Hydrophilus beschrieb, und ich es 
bei Forficula fand, nur vier betrigt. Der oben geschilderte Entwicke- 
lungsmodus des Tentoriums diirfte daher wohl als der typische anzusehen 
sein.” E 
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- 
torium 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 Savigny, 
— 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 vears of argu- 
ment, morphologists still disagree as to the number of somites composing 
the highly differentiated heads of insects. Kolbe (’90, p. 135) recognizes 
five, as follows :— 
1. Ursegment : Fihler, Augen, Oberlippe ; 
2. Ursegment : Oberkiefer oder Mandibeln (1. Kiefernpaar) ; 
3. Ursegment: Unterkiefer oder Maxillen (2. Kiefernpaar) ; 
4, Ursegment : Zunge oder Innenlippe (3. Kiefernpaar, verwachsen) ; 
5. Ursegment: Unterlippe (4. Kiefernpaar, verwachsen). 
Sharp (95, p. 87) says, “ Morphologists are not yet agreed as to their 
