318 PRIMITIVE SUCTORIAL MOUTH. 



to be some interesting embryological evidence, to which attention 

 has already been called in the preceding chapters. In a large 

 number of the larvae or embryos of the lower Vertebrates the 

 mouth has a more or less distinctly suctorial character, and is 

 connected with suctorial organs which may be placed either in 

 front of or behind it. The more important instances of this 

 kind are (i) the Tadpoles of the Anura, with their posteriorly 

 placed suctorial disc, (2) Lepidosteus larva (fig. 195) with 

 its anteriorly placed suctorial disc, (3) the adhesive papillse 

 of the larvae of the Tunicata. To these may be added the 

 suctorial mouth of the Myxinoid fishes 1 . 



All these considerations point to the conclusion that 

 in the ancestral Chordata the mouth had a more or less 

 definitely suctorial character 2 , and was placed on the 

 ventral 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. 



The 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 condition 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 Elasmobranchii is probably 

 to be explained by the fact that the members of this group 

 undergo an abbreviated development within the egg. 



1 The existing Myxinoid F'ishes are no doubt degenerate types, as was first clearly 

 pointed out by Dohrn ; but at the same time (although Dohrn does not share this view) 

 it appears to me almost certain that they are the remnants of a large and very primitive 

 group, which have very likely been preserved owing to their parasitic or 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. 



2 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. 



