774 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



mobranch eye in the treatise On Comparative Embryology, by 

 one of us 1 , will not fail to recognize that the folds of the retina 

 at the sides of the choroid slit, and the mesoblastic process 

 passing through this slit, are strikingly similar in Lepidosteus 

 and Elasmobranchii ; and that, if we are justified in holding 

 them to be an imperfectly-developed processus falciformis in the 

 one case, we are equally so in the other. 



Johannes Miiller mentions the absence of a processus falci- 

 formis as one of the features distinguishing Ganoids and Te- 

 leostei. So far as the systematic separation of the two groups 

 is concerned, he is probably perfectly justified in this course ; 

 but it is interesting to notice that both in Ganoids and Elasmo- 

 branchii we have traces of a structure which undergoes a very 

 special development in the Teleostei, and that the processus 

 falciformis of Teleostei is therefore to be regarded, not as an 

 organ peculiar to them, but as the peculiar modification within 

 the group of a primitive Vertebrate organ. 



SUCTORIAL Disc. 



One of the most remarkable organs of the larval Lepidosteus 

 is the suctorial disc, placed at the front end of the head, to 

 which we have made numerous allusions in the first section of 

 this memoir. 



The external features of the disc have been fully dealt with 

 by Agassiz, and he also explained its function by observations 

 on the habits of the larva. We have already quoted (p. 755) 

 a passage from Agassiz' memoir shewing how the young Fishes 

 use the disc to attach themselves firmly to any convenient 

 object. The discs appear in fact to be highly efficient organs of 

 attachment, in that the young Fish can remain suspended by 

 them to the sides of the jar, even after the water has been 

 lowered below the level at which they are attached. 



The disc is formed two or three days before hatching, and 

 from Agassiz' statements, it appears to come into use imme- 

 diately the young Fish is liberated from the egg membranes. 



We have examined the histological structure of the disc at 

 various ages of its growth, and may refer the reader to Plate 34, 



1 Vol. n. p. 414 [the original edition]. 



