CHAPTER IV 



THE EARLY VERTEBRATES AND THE FISHES 



The lowest of the vertebrates — if indeed it can be called a verte- 

 brate at all, seeing that it has no vertebrae — is the lancelet, 

 Amphioxus. The common species of this animal (there are some 

 eight in all) occurs in the sea off our own coasts, and is usually 

 to be found half buried in the sand or mud of the sea floor. It is 

 some two inches in length and has the shape of a laterally flattened 

 cigar, and one of its very obvious features is the arrangement of 

 the muscles in regular layers from front to back, in the same 

 manner as those of a fish. 



To describe some if its features in detail, the alimentary canal 

 bears a somewhat striking similarity to that of Balanoglossus. 

 There is a round, simple mouth, unprovided with jaws, and sur- 

 rounded by a number of projecting bristles. This leads into a 

 large pharynx, through the walls of which, on either side, pass a 

 large number of gill slits. The pharynx is not divided into an 

 upper and a lower canal, but there is a shallow groove along the 

 bottom which serves the same purpose as the food canal in Balano- 

 glossus. The remaining, digestive, part of the gut is practically 

 a simple tube, with a blind sac attached, representing the liver. 

 The gill slits do not open directly to the exterior, but into the 

 so-called peribranchial chamber, formed by the junction below 

 the body of two flap-like outgrowths, one from the upper part of 

 either side. This chamber opens to the outside by a single pore. 



Above the gut lies a straight, cylindrical rod of cartilage, 

 pointed at either end. This is the highly important structure 

 known as the Notochord, which is present in all the vertebrates, 

 although in the higher forms it is replaced during 'development 



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