GENERAL STRUCTURE. 



915 



there being a central pulp-cavity, surrounded by a layer of dentine 

 or vasodentine, which is covered with enamel. 



The teeth of Fishes present a greater amount of variation than is 

 found in any other class. They may be entirely absent, or may be 

 present on all the bones of the mouth, and also on the hyoids and 

 branchial arches, while they may be attached merely to the mem- 

 brane of the mouth-cavity. Very frequently they are attached by 

 anchylosis to the underlying bone or cartilage (fig. 838), but they 

 may be implanted in distinct sockets or alveoli. The dentine is usu- 

 ally distinguished from that of the teeth of higher Vertebrates by its 

 greater vascularity. The coating of enamel is 

 generally very thin ; but it is more developed 

 in the cutting-teeth of Sargus. Occasionally 

 (Dendrodus) radiating prolongations of the pulp- 

 cavity may penetrate the dentine from the centre 

 to the periphery, thus producing a structure like 

 that of the teeth of the Labyrinthodont Am- 

 phibians. There is generally a constant renewal 

 of the teeth of Fishes during the whole of life ; 

 but occasionally one set persists. 



Turning to the endoskeleton, and commenc- 

 ing with the vertebral column, we find that the 

 vertebrae can only be divided into a trunk and 

 a caudal series, and that there is a gradual pro- 

 gression in respect of ossification from the lowest 

 to the highest forms. Thus, in the Cyclostomi, 

 the notochord persists throughout life, and is 

 generally unsegmented, although rudimental 

 neural arches and spines are developed in Pet- 

 romyzon. The vertebral column of the Carti- 

 laginous Ganoids is very similar to that of the 

 latter; but in Bony Ganoids, Elasmobranchei, 

 and Teleostei paired cartilages, arising both 

 above and below the notochord, gradually sur- 

 round it, and thus form strongly amphiccelous 

 vertebral centra. In the Sharks these centra 

 exist without arches, but in the other orders 

 there are well-developed neural arches ; and in the trunk region 

 there are also lateral basal processes, which in the tail unite inferiorly 

 to form a haemal arch for the caudal artery, and develop a haemal 

 spine (fig. 839). There is a great tendency for the neural arches 

 to remain open superiorly ; and the only Fish in which the vertebrae 

 are not amphiccelous is the Ganoid Lepidosteus, in which they are 

 opisthoccelous. Only in the Chimeroidei and certain Elasmo- 

 branchei are there definite articulations between the vertebral 



-ha 



Anterior as- 

 pect of a caudal vertebra 

 of a Teleostean Fish ns, 

 Neural spine ; na, Neu- 

 ral arch ; z, Articular pro- 

 cesses ; ha, Haemal arch ; 

 hs, Haemal spine. (After 

 Giinther.) 



