37 o ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



is the presence of two types of organ which we have not met with hitherto 



external gill and cement-organ. The former, shown at the height of 



its development in Fig. 155, D, is a feathery projection from the hyoid 

 arch, richly supplied with blood by the second aortic arch which loops 

 out into it. It forms the main respiratory organ of the larva but com- 

 pletely disappears before the adult condition is reached. The cement- 

 or-un is a gland producing a sticky secretion by means of which the 

 young Polypterus hangs on to water-plants or other objects. It is 

 conspicuous in early larval stages (Fig. 155, A, c.o) but soon disappears 

 without leaving a trace behind. The organ contains a deep pit opening 

 at its tip and lined by glandular epithelium, and embryological study 

 brings out the remarkable fact that this glandular epithelium is originally 

 a part of the endodermal lining of the alimentary canal which grows out 

 into a pocket and becomes continuous with the outer ectoderm while it 

 loses its primitive connexion with the alimentary canal. There is clearly 

 some interesting evolutionary secret underlying this peculiar mode of 

 development. 



More closely allied to the modern teleosts than are Polypterus and 

 Calamichthys are the few more or less archaic genera which are grouped 

 together as the GANOIDEI or actinopterygian ganoids. These include the 

 ordinary Sturgeons (Acipenser Fig. 156), which make their way up the 

 rivers of the Northern Hemisphere to spawn, and the ovary or " roe " of 

 which is well known as caviare ; the Shovel-bill Sturgeon (Polyodon) of 

 the Mississippi ; and Psephurus of the Yang-tze-Kiang. The Garpike 

 (Lepidosteus Fig. 157) and the Bowfin (Amia Fig. 158) of North 

 American fresh-water lakes are also included in the Ganoidei. 



The chief interest of these fishes lies in their retention of various 

 features characteristic of Elasmobranchs and Crossopterygians. In the 

 Sturgeons the mouth retains its primitive ventral position : in all the 

 tail is more or less markedly heterocercal. In the Sturgeons the skeleton 

 is cartilaginous, except that there are well-developed plates of bone in 

 the skin and that the cartilaginous cranium is reinforced by bony plates 

 applied to its surface. In Amia and Lepidosteus the cartilage of the 

 skeleton becomes as extensively replaced by bone as in a typical teleost. 

 So far as the vertebral column is concerned ossification has gone further 

 in Lepidosteus than in any other fish, the centra having come to fit closely 

 into one another, the convex anterior face of the centrum fitting into 

 the concave posterior face of the next vertebra behind it (opisthocoeleus) 

 whereas in the teleost the centra are amphicoelous, with intervening spaces 

 occupied by persistent notochord. In Lepidosteus there are ganoid scales 



