108 Biology in America 



extreme sensitiveness to Ijnjlit, to avoid which it quickly with- 

 draws into its burrows in the sand. At the anterior end of 

 the body arc a number of delicate fingers or tentacle-like 

 processes surrounding the mouth, wiiich is ventral in position 

 and which opens into the pharynx, whose walls are perforated 

 by numerous gill slits, opening into a common branchial cham- 

 ber or atrium. These are, briefly put, the principal features 

 of this very curious and interesting little creature. In com- 

 mon with the vertebrates it has a hollow, dorsal nerve cord, 

 a notocliord and gill slits, w^hile the absence of vertebrae and 

 of anything resembling jaws, and its extensive segmentation 

 relate it to invertebrates. Its organization is however dis- 

 tinctly more vertebrate in character than is that of either the 

 tunicate or Balanoglossus. 



The first of the vertebrates proper however are the cyclo- 

 stomes, so named from their circular mouths, which lack jaws. 

 They include both marine and fresh water forms and are 

 dangerous parasites of fish, attaching themselves to the latter 

 by means of the suctorial mouth, and rasping away the 

 scales with their powerful tongue, armed with numerous horny 

 teeth, and sucking the blood and soft tissues until their victim 

 is destroyed. The hagfish Bdellostoma of the California 

 coast is an example. This is an eel-like creature which is 

 persona non grata to the Chinese fishermen, entangling their 

 nets and destroying their fish. One individual in a pail of 

 water will quickly convert it into a jelly-like mass due to 

 the abundant slime secreted. In these animals we find the 

 first typically vertebrate structure, namely the vertebral 

 column^ incorrectly called the "back bone," since it is not 

 necessarily bony, Ibut may consist of cartilage, and in some 

 instances is not a continuous structure at all, but consists 

 merely of a series of disconnected cartilaginous pieces, par- 

 tially surrounding the nerve cord at the base of which lies 

 the notochord. Whether or not the cyclostomes are primitive 

 or degenerate types is a bone of contention among zoologists. 

 In their i)rimitive vertebiu', persistent notochord, extensive 

 segmentation of nerves and muscles and numerous gills their 

 primitive character is clearly indicated ; but in their suctorial 

 mouth, and poorly developed eyes there is evidence of degen- 

 eration, correlated perhaps with their parasitic habits. 



Not until we reach the true fishes do we find fully developed 

 the vertebrate plan of structure, with a complete vertebral 

 colunm built around and replacing the degenerating notochord, 

 and surrounding the nerve cord; with paired upper and lower 

 jaws and paired fins, and a fully developed brain case or 

 skull. The origin of most of these structures is shrouded in 

 mystery, and unless paleontology comes to our aid, re- 



