78 PRIMITIVE ANIMALS 



since it undoubtedly represents in many of its 

 features a primitive or ancestral form of Chordate. 

 The Lancelet (Fig. 17) is a little cigar-shaped marine 

 animal, attaining an inch or two in length ; it swims 

 with great activity and when alarmed burrows with 

 extraordinary rapidity into fine sand. It is found in 

 small numbers round the English coasts, but with 

 much greater frequency in the Mediterranean and 

 tropical seas. It possesses the essential organization 

 of a very simple fish, but the simplicity of its 

 structure, especially as regards sense organs, brain, 

 skeleton and vascular system is carried to such an 

 extent; that the actual resemblance to a fish is ex- 

 tremely shadowy. The skeleton is represented solely 

 by the long notochord which stretches, as a flexible 

 gelatinous rod from the tip of the snout, along the 

 back, to the tip of the tail. There are no skull, or 

 jaws, or paired limbs, though there is a fin running 

 along the back and expanded at the tail. Thus as 

 compared with a fish the skeleton is in a rudimentary 

 state, being represented solely by the notochord. This 

 organ, which is always present in the embryo Verte- 

 brate, is invariably replaced in the adult Vertebrate, 

 either partially or entirely, by the segmented vertebral 

 column or spine, which grows over and encloses the 

 nerve tube. The nervous system is in the form of 

 a hollow tube running along the back, just above 

 the notochord. In the head it gives off two pairs of 



