otic capsule 



notochord 



pericardial cartilage 



last branchial opening 



nosoh/pophyseal opening 

 posterior lateral cartilage 

 posterior tectal cartilage 



nasal capsule 



anterior tectal cartilage 

 annular cartilage 



B 



subocular arch 

 piston cartilage 

 cornual cartilage 'styloid cartilage (hyoid arch) 



Figure 5-24. Head and visceral skeletons of the lamprey (partly after Marinelli and Strenger, 

 1 954). A, entire head and visceral skeleton as seen in lateral view; B, anterior port, enlarged for 

 detail, OS seen in lateral view. 



Lamprey The lamprey head skeleton consists of an endo- 

 cranium and an extensive "visceral" skeleton (Figures 

 5-24, 5-25). The endocranium includes a single nasal cap- 

 sule, paired otic capsules, and the side walls and floor of the 

 cranial cavity. The otic capsules are joined above the cra- 

 nial cavity by a synotic tectum. The notochord is much 

 constricted betv/een the otic capsules; it rapidly expands to 

 its spinal proportions posteriorly. It is exposed in the floor 

 of the cranial cavity and thinly covered ventrally by the 

 basis cranii. The basal plate projects posteriorly along the 

 notochord on either side. The anterior tip of the notochord 

 is exposed at the posterior margin of the large hypophyseal 

 fenestra. Anterior to the fenestra is a broad basitrabecular 

 plate lying below the large midline nasal capsule. 



Behind the neurocranium one or more of the "occipital" 

 arch elements can be viewed as representing the occipital 

 segments of the gnathostome head skeleton. The ninth and 

 tenth cranial nerves exit behind the otic capsule, as does an 



occipitospinal nerve. The next segmental nerve perforates 

 the base of the first neural arch. 



The side wall of the neurocranium is joined to the otic 

 capsule at three points: above, anteriorly, and ventroante- 

 riorly. The dorsal opening formed by these connections is 

 filled with connective tissue. The more ventral, larger open- 

 ing is used by the trigeminal, abducens, and trochlear 

 nerves. Anterior to this opening is a large optic foramen and 

 just behind this a small oculomotor foramen; these may be 

 confluent. Ventroanterior to the optic foramen is a small 

 arterial (orbital or facial) foramen. 



The otic capsule has a large internal auditory fenestra 

 for the common root of the seventh and eighth nerves and 

 above this a good sized endolymphatic foramen. The facial 

 nerve leaves the eighth just inside the capsule and passes 

 forward and downward through a small canal opening be- 

 tween the capsule and the cranial base just behind the 

 trigeminal foramen. 



AGNATH FISHES 



129 



