. Chordate morphology. Morphology (Animals); Chordata. 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 hemi
. Chordate morphology. Morphology (Animals); Chordata. 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 arte
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