418 HISTORICAL GEOLOGY. 



living Ceratodus to survive in the muddy pools of dried-up streams in Australia. The 

 pectoral fins are a pair of slender filaments in Lepidosiren ; thickened paddle-shaped fins 

 vi^ith a jointed axis in Ceratodus, and have a thiclvened axis in Plianeropleuron and other 

 ancient genera. 



Tkleosts. — The Teleosts include nearly all of the modern fishes except the Sharks 

 and Rays and the few existing Ganoids. They are closely allied to the Ganoids, through 

 the existing Amia and related forms. They have a bony skeleton, as implied in the name 

 (from rAeos, perfect, o^riov, bone) ; and the gills are free. In the absence of a valve 

 between the intestine and stomach they are unlike the Ganoids and Sharks and inferior 

 to them in type of structure. The body usually has scales, which are either cycloid 

 (Fig. 369), or ctenoid (Fig. 370), the latter term referring to the toothed or spinous 

 margin, and coming from the Greek for comh ; but in some kinds there are bony plates. 



Cyclostomes (Marsipobranchs) or Lampreys, etc., having a simple cartilaginous noto- 

 chord ; no jaws ; mouth a circular opening for suction, usually with conical teeth on its 

 inner surface ; gills pouch-like ; no fins. 



6. Leptocardians. 



Amphioxus (or Branchiostoma) : embryonic forms having a simple fibrous notochord 

 in place of a vertebral column ; cranium and distinct brain lacking ; heart tubular ; gill 

 a saccular dilation of the oesophagus ; no jaws ; the organs of the senses partly wanting. 

 The species are all small. 



Relation of Vertebrates to Invertebrates. — The Invertebrates are widely separated in 

 character from the Vertebrates. The nearest group to Fishes among them is that of the 

 Ascidians or Tunicates, formerly referred to the class of Mollusks and regarded as not 

 higher among species than the Oyster, all special organs of locomotion being absent, 

 and little to be seen in an outside view but a bag with two holes for the passage of water 

 — inward at one hole and outward at the other. But the animals are little like Mol- 

 lusks structurally, and have certain peculiarities in their embryonic development which 

 manifest a relationship to the Vertebrates. In the young stage some of them have a 

 resemblance in form and somewhat in organs to the tadpole of a Frog and the embryo-like 

 fish, Amphioxus. The Ascidians are consequently regarded as related either to a prototype 

 form of Vertebrate, or else to a degenerate form in the Vertebrate series. The relation is 

 briefly presented in a well-illustrated article by Lankester entitled Vertebrata, contained 

 ni the 24th volume of the Encyclopoedia Britannica. 



Invertebrates. 



The old subdivisions of the Invertebrates are : Protozoans ; Radiates, including 

 Polyps, Hydrozoans, and Echinoderms ; Mollusks, including Mollusks, Bryozoans, and 

 Brachiopods ; Articulates, including Worms, Crustaceans, and the terrestrial kinds, 

 Myriapods, Arachnids, and Insects. Through embryological study it has been proved 

 that true Protozoans are one-celled in all stages, the embryo cell undergoing no subdivis- 

 ion (that is, segmentation) in the development. In Sponges, on the contrary, while there 

 is much external resemblance to Protozoans, the germ-cell undergoes segmentation as in 

 higher species, and hence there is a nearer relation to Polyps than to the simple Protozoans. 

 It has also been found that Brachiopods are about as nearly related to Worms as to Mol- 

 lusks ; that Echinoderms are more nearly related to Worms than to Polyps and Hydro- 

 zoans, notwithstanding the radiate arrangement of the external parts ; that Polyps and 

 Hydrozoans (Medusae) are closely related, and as they have the common character of 

 a single opening to the interior cavity, they now are called Ccelenterates, from /cotXos, hol- 

 loio, and fvrepov, intestine. 



