SEGMENTATION OF THE HEAD 15 



segmental phragma between the maxillary and labial segments. 

 The hypothesis that this suture marks the division between those 

 two segments is borne out by the positions of the maxillary and 

 labial articulations with reference to it. 



After the gnathal segments became added to the protocephalon, 

 it appears probable that their sternal parts lost their identities. 

 From the fact that the ventral adductor muscles of all the mouth- 

 parts have their origins on a pair of apophyses, arising from the 

 base of the hypopharynx in Chilopods and most Apterygota, 

 Snodgrass concludes that at least part of each of their sterna 

 has become incorporated into the hypopharyngeal region — a 

 conclusion which is supported to some extent on embryological 

 grounds. From a comparative study of Crustacea, Myriapoda 

 and Apterygota he has advanced an ingenious theoretical inter- 

 pretation of the origin and evolution of the insect tentorium 

 (Fig. 6). Without going fully into his lines of reasoning, it may 

 be said that the beginnings of that structure are to be seen in the 

 hypopharyngeal apophyses previously alluded to. From their 

 original position, at the sides of the hypopharynx, it appears that 

 they have migrated outward in the ventral"^wall of the head to 

 the lateral edges of the cranium, but retain the original muscle 

 attachments. In Odonata and Ephemeroptera these apophyses 

 come to lie in the sub-genal sutures, but in all other Pterygota 

 they have proceeded further to take up their usual position on 

 either side of the clypeo-frontal suture. The hypopharyngeal 

 apophyses have, according to this interpretation, become the 

 anterior arms of the tentorium. The posterior arms are invagina- 

 tions in the lower extremities of the post-occipital suture, which 

 unite with the anterior arms to form the typical four-branched 

 skeletal structure : they are undeveloped, it may be added, in 

 Myriapoda and most Apterygota. The dorsal arms arise as 

 processes of the anterior pair, and may subsequently acquire 

 secondary connection with the cranial wall. The condition of the 

 tentorium in Machilis appears to be largely intermediate between 

 that found in most Apterygota and Pterygota, since the anterior 

 and posterior elements are present, but still independent of each 

 other. 



