NEAL : NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 263 



homologized with the mouth of Ascidians 1 2. Is it or is it not homol- 

 ogous, either wholly or iu part, with the mouth of Craniota 1 Upon the 

 answer to the former question would seem to depend the settlement of 

 the question whether the mouth of Amphioxus may or may not be re- 

 garded as a visceral cleft, for there is no reason to believe that the 

 mouth of Ascidians represents a pair of visceral clefts. Notwithstanding 

 that Willey appears to have in the asymmetrical mouth of Amphioxus 

 strong evidence in favor of his homology, which has also met the 

 approbation of Marshall ('93), I consider the different relation of the 

 Tunicate and Vertebrate mouth to the brain vesicle a very serious ob- 

 jection to his theory. Furthermore, the presence of a preoral intestine 

 in Vertebrates, which in Squalus extends (morphologically) anterior to 

 the infundihulum, — even to the neuropore, as does the intestine of 

 Tunicates, — leads me to agree with Beard, Kuptier ('88 and '91), and 

 van Wijhe ('94), that in the present mouth of Vertebrates we have a 

 neostoma, and also that a palceostoma homologous with that of Tunicates 

 must be sought in an anterior opening of the preoral intestine. Kupf- 

 fer finds evidence of this palceostoma in the ectodermic invagination of 

 the hypophysis towards the " Preoraldarm," while van Wijhe finds 

 it in Amphioxus, as stated in the Table, in the actual opening of 

 the preoral intestinal diverticulum of the left side as the preoral pit 

 (Raderorgan). 



Waiving the question as to which, if either, of these theories is cor- 

 rect, I regard the mouth of Ascidians as opening at the morphologically 

 anterior end of the alimentary canal ; for there appears to me noth- 

 ing in the literature upon Tunicates to show the presence of a preoral 

 intestine in these forms. The mouth of Appendicularia, which has no 

 " preoral lobe," (though homologized by Willey with the preoral intes- 

 tinal diverticula of Amphioxus and the premandibular cavities of Crani- 

 ota,) has a terminal position.^ According to Willey the method of 

 formation of the preoral lobe in those Ascidians possessing such is as 

 follows (p. 218): "When the larva first hatches, the entoderm and 

 ectoderm are in contact with one another at the anterior extremity of 

 the body, just as they are in the earlier stages. Soon, however, the 

 ectoderm, with the adhering papillae, springs away from the endoderm 

 at this point, leaving a space into which the two lateral mesodermic 



1 Willey ('94, p. 277) writes: "Whatever the truth maj' be as to tlie precise 

 systematic position and phylogonetic value of Appendicularia, one thing, to Miy 

 mind, remains absolutely certain, namely, that it has descended from a form which 

 possessed a prjeoral lobe, and that it has secondarily lost that structure." 



