STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 775 



figs. 1 1 and 13, and Plate 37, figs. 40 and 44. The result-of-our 

 examination has been to shew that the disc is provided with 

 a series of papillae often exhibiting a bilateral arrangement. 

 The papillae are mainly constituted of highly modified cells of 

 the mucous layer of the epidermis. These cells have the form 

 of elongated columns, the nucleus being placed at the base, and 

 the main mass of the cells being filled with a protoplasmic reti- 

 culum. They may probably be regarded as modified mucous 

 cells. In the mesoblast adjoining the suctorial disc there are 

 numerous sinus-like vascular channels. 



It does not appear probable that the disc has a true sucking 

 action. It is unprovided with muscular elements, and there 

 appears to be no mechanism by which it could act as a sucking 

 organ. We must suppose, therefore, that its adhesive power 

 depends upon the capacity of the cells composing its papillae to 

 pour out a sticky secretion. 



MUSCULAR SYSTEM. 



There is a peculiarity in the muscular system of Lcpidostens, 

 which so far as we know has not been previously noticed. It is 

 that the lateral muscles of each side are not divided, either in 

 the region of the trunk or of the tail, into a dorso-lateral and 

 ventro-lateral division. 



This peculiarity is equally characteristic of the .older larvae 

 as of the adult, and is shewn in Plate 41, figs. 67, 72, and 73, 

 and Plate 42, figs. 74 76. In the Cyclostomata the lateral 

 muscles are not divided into dorsal and ventral sections ; but 

 except in this group such a division has been hitherto considered 

 as invariable amongst Fishes. 



This character must, without doubt, be held to be the indica- 

 tion of a very primitive arrangement of the muscular system. 

 In the embryos of all Fishes with the usual type of the lateral 

 muscles, the undivided condition of the muscles precedes the 

 divided condition ; and in primitive forms such as the Cyclosto- 

 mata and Amphioxus the embryonic condition is retained, as it 

 is in Lepidostcus. 



