Acrania. 151 



Other Tunicates. Some tunicates are minute and free- 

 swimming by means of a vibratile tail. Other small forms 

 are barrel-shaped, and exhibit a marked " alternation of 

 generations." Many of the tunicates live and multiply by 

 budding in colonies. 



SUBBRANCH VERTEBRATA. 



The Lowest Vertebrate. To the beginner it would seem 

 easy to determine whether or not an animal has a back- 

 bone, and so to decide whether it is a vertebrate or an 

 invertebrate. But let us take a glance at what is by many 

 authors regarded as the simplest of the vertebrates. 



The Lancelet. The lancelet (Branchiostoma, or Amphi- 

 oxus) is fishlike in form and general appearance, only two 



FIG. 98. DIAGRAM OF LANCELET. 



Above (dotted) is the nervous system; below it (cross-lined) the notochord; the mouth is sur- 

 rounded by a circle of tentacles; below the notochord is a row of gill slits; the vent is 

 near the posterior (right) end below. From Kingsley's Zoology. 



or three inches long, and nearly transparent. It is marine, 

 being found in warm waters. Specimens are taken along 

 the south Atlantic coast. The lancelet has a notochord 

 extending to the anterior end, or the snout. There is a 

 nerve cord along the dorsal side of the notochord, but the 

 anterior end is hardly well enough developed to deserve 

 being called a brain. It has blood tubes, but no heart. 

 There is a tail fin, but no limbs, not even paired fins. The 

 mouth is surrounded by a circle of fringelike tentacles. 

 Back of the mouth extends the capacious pharynx, whose 

 walls are perforated by numerous ciliated gill slits. At the 



