. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. RESPIRATORY ORGAN'S. 23. of the first, are lost. Their presence in this group can only be explained as inheritances from branchiate ancestors. The first gill pouch in the anura and the higher groups form the Eusta- chian tube (see ear). In elasmobranchs and some ga- noids the anterior visceral cleft is smaller than the others, and opens on the top of the head. This spir- acle bears well-developed gills in the lowest sharks (notidanidae), but in others it may have but a vascular network in its walls. In ganoids and embryonic
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. RESPIRATORY ORGAN'S. 23. of the first, are lost. Their presence in this group can only be explained as inheritances from branchiate ancestors. The first gill pouch in the anura and the higher groups form the Eusta- chian tube (see ear). In elasmobranchs and some ga- noids the anterior visceral cleft is smaller than the others, and opens on the top of the head. This spir- acle bears well-developed gills in the lowest sharks (notidanidae), but in others it may have but a vascular network in its walls. In ganoids and embryonic teleosts it has a gill-like structure; but it is here termed a pseudobranch, since it receives arterial blood from the opercular gill. The opercular gill is a secondary and ectodermal structure developed on the inner or posterior face of the operculum (see below). In the typical elasmobranchs the interbranchial septa extend to the outside of the body, and the gill clefts open directly to the exterior, either on the sides of the neck (selachii, Fig. 26) or on the ventral surface (raia;). In the cyclostomes, Myxine excepted, there is also a separate opening for each gill cleft. In the holocephali a fold of skin on either side grows back over the gill clefts, thus en- closing a space into which these empty, and which in turn connects with the exterior by a slit-like gill opening be- hind. In the ganoids and teleosts the same relations occur; but in these the fold, known as the operculum, has a cartilaginous or bony internal skeleton. In the amphibia the opercular fold is also found, but Fig. 24. Section through the head of a pig embryo mm. long, showing the gill slits (i, 2, 3,) closed by a thin wall. At the left a small portion enlarged. E, Eustachian cleft; H, hyphophysis; AI, man- dibular cleft; P, pharynx. Fig. 25. Tadpole of frog, showing at g the external gill Please note that these images are extracted from scanned page images that may have been digitally enha
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