THE VERTEBRATA 145 



of which have been recently discovered, while 

 others have been known for some time, although 

 their affinities were not at first recognised. 

 Among these the most remarkable are sessile 

 forms which have received the names respectively 

 of Cephalodiscus and Rhabdopleura. Both produce 

 buds and form a colony, and in both a notochord 

 has been distinguished. The former was procured 

 from the Straits of Magellan, while the latter 

 makes its dwelling-place in a nearer region, hav- 

 ing been found off the Shetland Islands, and off 

 the Lofoden Islands. Cephalodiscus, which is a 

 very curious creature, receives its name from a 

 disc placed at the head end. The use of this 

 structure is believed to be as follows. The units 

 of the colony live inside a common system of 

 tubes, which they secrete; each unit, when adult, 

 is independent, and can move about inside the 

 tubes; the disc is used as a means of attachment 

 to successive spots of the tube-wall, as the ani- 

 mal wanders from place to place. Above the disc 

 are twelve plume-like tentacles covered with cilia, 

 which create a current in the water surrounding 

 the head, and wash food particles into the mouth. 

 That these creatures are but distant relations 

 of the true vertebrates is a fact expressed by the 

 names under which they are grouped in classifi- 

 cation. Those forms which we have just de- 

 scribed have received the name of Hemichordata 

 that is to say, Chordata which have but half a 

 notochord, since the notochord is very restricted 

 in extent; while the Ascidians are grouped under 

 the name of Urochordata, or Chordata which 

 only possess a notochord in the tail. The name 

 of Adelochorda, " with an obscure chord," is 

 sometimes applied to the Hemichordata. 



