SUB-KINGDOM I.— VERTEBRATA. 



CLASS VII.—PROTOGHORDA. 

 By W. Gabstang, M.A., F.Z.S. 



SUB-CLASS I.— CEPHALOCHOEDA. 



The group Cephalochorda contains a number of small marine animals which 

 resemble tishes in many respects, but are much more lowly organised than 

 even the simplest of the finny tribe. They breathe like fishes by taking 

 water into their mouths and passing it out through a series of slit-Uke holes 

 in the sides of their throats (giU-slits). The number of these gill-slits is much 

 greater than in fishes, frequently amounting to more than a hundred. A 

 supporting rod, the notochord, the forerunner of the backbone of higher 

 forms, extends along the back of the animal from head to tail, and the tubu- 

 lar nervous system lies above it. There are no true side-fins, but a pair of 

 ridges along the under side of the body of the young animal possibly re- 

 presents them. In the very young animal the gill-slits open directly to the 

 outside, but at an early period the fin -like ridges, which have just been 

 mentioned, close over the slits and unite with one another on the under side 

 of the body, leaving merely a small hole or water-pore for the outflow of the 

 water. These little animals are capable of extremely rapid movements, 

 w-riggling their lancet-shaped bodies from side to side after the manner of an 

 eel. They have no jaws, and depend for their food upon the microscopic 

 plants and other organisms contained in the stream of water constantly 

 passing through their mouths. This current of water is set up by the action 

 of innumerable fine whip-like lashes which beset the sides of the giU-slits, and 

 by their united action drive the water outwards into the surrounding water- 

 cavity, and so to the exterior through the water-pore. New water is con- 

 stantly streaming into the mouth to take the place of the water driven out, 

 and the food-particles it contains are incessantly extracted from it by means 

 of the slimy coating of the walls of the throat, to which they adhere. Another 

 set of microscopic lashes then drives the entangled food-particles down the 

 proper channel to the animal's stomach, where they are digested in the 

 ordinary way. 



The lancelet (Amph.i(iyiis lanrrolntus) is the only representative of the 

 group in the Mediterranean and North Atlantic seas. It is a semi-transpar- 

 ent little creature, from one to two 

 inches in length, compressed from side 

 to side, pointed at both ends, provided 

 with a circle of small stiff tentacles round 

 the mouth, and with a distinct tail-fin 

 F-. 1 T„^ T .x,™.„, /• I I • ™ 7 behind. It lives in shallow water, in 



latus). beds of loose sand, into which it burrows 



with lightning-like rapidity. It lies 

 obliquely on its back in its sandy bed, its head and mouth alone protruding 



526 



