The arterial pattern of this species seems Uttle modified 

 in terms of a hypothetical ancestral type. In other sharks, 

 Squalus or Muslelus, the external carotid is only hypobran- 

 chial in distribution, and there is no connection with the post- 

 orbital stem. The shark differs from the actinopterygian or 

 dipnoan in that the hyoid vessels are like those of the 

 branchial arches (only Amia approaches this). 



Hydrolagus is much like Squalus in that the hyoid aortic 

 arch is retained and forms afferent and efferent divisions as- 

 sociated with a hemibranch. The efferent hyoid connects 

 with the lateral dorsal aorta, and the mandibular efferent 

 (pseudobranchial artery — there is no pseudobranch) enters 

 the endocranium to supply the brain. An orbital artery 

 arises from the efferent hyoid connection with the dorsal 

 aorta. This penetrates the palatoquadrate and sends branches 

 anterolaterally to the maxilla and snout and posteriorly to 

 the otic region. 



Cyc/ostomes The head arteries of the lamprey have been 

 described but are not generally known. The external ca- 

 rotid arises from the efferent vessels of the first two branchial 

 arches (Figure 11-25). It gives rise to a thyrolaryngeal 

 branch and passes forward medial to the afferent stem (on 

 either side). At the anterior margin of the branchial skeleton, 

 a branch extends forward and inward to the roof of the 

 mouth. A short distance anteriorly, the main stem gives off 

 an external branch which passes up behind the muscles of 

 the mouth. This branch gives off branchlets to the muscles 

 in front and behind it and continues up behind the eye, 

 where it supplies the musculature. The external carotid 

 stem continues forward branching to the muscles of the 

 rasping organ. There are several separate efferent branches 

 serving the ventral wall of the branchial basket. 



The internal carotids arise from the dorsal aorta in the 

 region of the otic capsule. Each receives a hyoid efferent 



ophthalmic artery 



posterior cerebral ^^ st°pedial-hyomandibular efferent 

 , occipital 



brachial artery 



infraorbital 



mesenteric 

 coelioc artery 



2nd branchial afferent 



1st branchial efferent 



efferent hyoid 

 afferent pseudobranchial 



A LAMPANYCTUS 



stapedial 

 efferent pseudobranchial artery to eye I pseudobranch 



orbitonasal artery 



B OPHIODON 



along anterior edge of preopercle 

 brachial artery 



coelroc artery 



efferent hyoid bronchiostegal 



Figure 1 1-29. Anterior arteries of two teleosts, tompanyctus and Ophiodon elongatus, as seen in 

 semidiagrammatic lateral view. (B after Allen, 1905) 



CIRCULATION IN THE HEAD 



359 



