SKULL 



219 



The remaining anterior skull elements are principally skeletal 

 supports for the roof and walls of the buccal funnel. The roof 

 is supported by an extended anterior dorsal cartilage, which is 

 overlapped behind by the ethmoid cartilage, while the circular 

 margin of the funnel is strengthened by a large ring-like annular 

 cartilage. On each side of the latter there is a slender, rod-like, 

 styloid cartilage, and above the latter a small anterior lateral 

 cartilage. All these "cartilages are usually termed labial cartilages, 

 and it is at least possible that they possess representatives in the 

 similarly named structures of the Dog-Fish and the larvae of 

 some of the tailless Amphibia. It must not be forgotten, how- 

 ever, that the annular cartilage bears some resemblance to the 

 ring of cartilage which encircles the lips of the buccal cavity in 

 Amphioxus. 



The complex supporting skeleton of the gill-sacs forms a basket- 

 like structure. It consists on each side of nine unsegmented, 

 irregularly curved, and slightly branched cartilaginous rods, 

 situated in the outer margins of the inter -branchial septa, 

 directly internal to the skin. The first lies directly behind the 

 styloid process (hyoid arch), the second and third in front of and 

 behind the first gill-sac, and of the remainder one lies just behind 

 each of the six succeeding gill-openings ; above and below each 

 gill-aperture the rods are connected by longitudinal bars, and also 

 in the median ventral line by a pair of similar partially united 

 bars. The dorsal ends of the rods are also connected on each side 

 by another longitudinal bar, which runs alongside the notochord 

 and in front blends with the chondrocranium. The rods forming 

 the last pair are continuous with a cup-like cartilage, supporting 

 the lateral and hinder walls of the pericardium. 



This singular branchial basket undoubtedly bears a superficial 

 resemblance to the branchial arches of Fishes, but in any comparison 

 of the two structures it is well to bear in mind that the branchial 

 rods of the Lamprey are situated along the outer edges of the 

 inter-branchial septa, and are therefore external to the gill-sacs 

 and branchial arteries, and further, that they are developed in 

 the somatic mesoblast of the embryonic protovertebrae, whereas 

 true branchial arches are situated at the inner margins of the 

 septa, internal to the gill-clefts and branchial arteries, and have 

 their origin from the splanchnic layer of the mesoblast. So far 

 as their position is concerned, the rods agree rather with the 



