264 



PRIMITIVE SUCTORIAL MOUTH. 



All these considerations point to the conclusion that 

 in the ancestral Chordata the mouth had a more or less 

 definitely suctorial character^ and was placed on the ven- 

 tral surface immediately behind the praeoral lobe; and 

 that this mouth has become in the higher types gradually 

 modified for biting purposes, and has been carried to the 

 front end of the head. 



Tlie mouth in Elasmobranchii and other Vertebrates is originally 

 a wide somewhat rhomboidal cavity (fig. 28 G) ; on the development 

 of the mandibular and its maxillary (pterygo-quadrate) process the 

 opening of the mouth becomes narrowed to a slit. The wide con- 

 dition of the mouth may not improbably be interpreted as a remnant 

 of the suctorial state. The fact that no more definite remnants of 

 the suctorial mouth are found in so primitive a group as the Elas- 

 mobranchii is probably to be explained by the fact that the members 

 of this group undergo an abbreviated development within the egg. 



While the emUryological data appear to me to point to the existence of a 

 primitive suctorial mouth, very different conclusions have been put forward 

 by other embryologists, more especially by Dohrn, which are sufficiently 

 striking and suggestive to merit a further discussion. 



As mentioned above, both Dohrn and Semper hold that the Vertebrata 



are descended from Chfetopod- 

 like forms, in which the ventral 

 surface has become the dorsal. 

 In consequence of this view 

 Dohrn has arrived at the follow- ' 

 ing conclusions : (1) that primi- 

 tively the alimentary canal per- 

 forated the nervous system in 

 the region of the original oeso- 

 phageal nerve-ring ; (2) that 

 there was therefoi"e an original 

 dorsal mouth (the present ven- 

 tral mouth of the Chsetopoda) ; 

 and (3) that the present mouth 

 was secondary and deri\'ed from 

 two visceral clefts which have 

 ventrally coalesced. 



A full discussion of these 

 A'iews' is not within the scope 



— -sd 



op 



Fig. 195. Ventbal view op the head of a 

 Lepidosteiis embryo shortly befobe hatching, 

 TO shew the large suctorial disc. 



m. mouth; op. eye; sd. suctorial disc. 



semiparasitic habits; much in the same way as many of the Insectivora have been 

 preserved owing to their subterranean habits. I am acquainted with no evidence, 

 embryological or otherwise, that they are degraded gnathostomatous forms, and the 

 group probably disappeared as a whole from its incapacity to compete successfully with 

 Vertebrata in which true jaws had become developed. 



1 I do not conceive that the existence of suctorial structures necessarily implies 

 parasitic habits. They might be used for various purposes, especially by predaceous 

 forms not provided with jaws. 



■■' For a partial discussion of this subject I would refer the reader to my Monograph 

 on Elasmobranch Fishes, pp. 165 — 172. 



