8 Proceedings of the Royal Irish Academy. 



A muscle {R), which has its origin upon the plate of the suhmentum, is 

 inserted upon a slight ridge, which is coincident with the distal edge of the 

 mentum, its action being to pull the latter forwards. A muscle {D') from the 

 mentum and a muscle [E') from the posterior edge of the central plate of the 

 tentorium (beside the origin of V, Plate I) are both inserted at the base of 

 the palpiger, moving it slightly perhaps, but more probably working the end 

 of the labium as a whole. A long, slender muscle {F), coming from the ten- 

 torium with E', also the muscles ^and IT, move the first segment of the 

 palp. The muscle L' moves its second segment, and N' and M its ter- 

 minal segment. The lacinia is bent back by the muscle A', being restored 

 by the elasticity of the unjointed cuticle in front; muscles i?'and C bend 

 back the galea, which is restored in a like manner. As has been mentioned 

 in the account of the tongue, a pair of muscles pass from the labium, above 

 the mentum, to the sides of the hypopharynx. 



VerhoefF ('05), as previously stated, regards the labium as the second pair 

 of mouth appendages, the maxillae, according to him, belonging to a succeeding 

 segment. His views are based to a great extent upon the nature of the 

 mentum and submentum. These he regards, not as fused portions of the 

 labial appendages, but as the sterna of two of the cephalic segments. He is 

 convinced that the mentum represents the sternum of the labial segment, the 

 submentum that of the maxillary segment. To account for this he supposes 

 that a shifting of the maxillae has occurred, from their primitive position 

 behind the labial appendages to their present situation anterior to the latter. 

 He lays stress upon the close relations which appear to him to exist between 

 the cardo and the submentum ; but even if they were united, it would hardly 

 be safe to draw conclusions as to their primitive connexion, as fusion between 

 neighbouring segments is of such common occurrence. 



Then all the muscles passing to the maxilla from the tentorium and 

 epicranial vault are anterior to the two pairs of muscles that go from the 

 tentorium to the labial palpiger and palp. This demands the almost total 

 disappearance of those primitive labial muscles which it is reasonable to sup- 

 pose, on Yerhoeff' s theory, at one time did pass to the head in front of those 

 from the maxillae. 



Judging from the figures given by Miall and Denny, there is nothing in 

 the arrangement of the tracheal or nerve supply suggestive of such a profound 

 disturbance in the primitive arrangement of the limbs. Though the ^dews 

 that have hitherto been put forward regarding the homologies of mentum and 

 submentum may well be criticized, yet the theory substituted by Verhoeff 

 appears to have far less basis in actual fact, and, by reason of its highly specu- 

 lative character, it will most probably be adopted by few, if any, zoologists. 



