LANCELET. 



113 



LEPTOCAHDII. 



The Lancelet {Branchiostoma or Amphioxus) has been long 

 considered to be the lowest in the scale of fishes, but it lacks so 

 many characteristics, not only of this class, but of the Vertebrata 

 generally, that it is better referred to a separate class, the chief 

 characters of which are as follows : — Skeleton membrano-cartila- 

 ginous and notochordal, ribless. No brain. Pulsating sinuses in 

 place of a heart. Blood colourless. Respiratory cavity confluent 

 with the abdominal cavity ; branchial clefts in great number, the 

 water being expelled by an opening in front of the vent. Jaws none. 

 Six species of Lancelet [Branchiostoma, fig. 101) are known, and 



Fig. 101. 



Lancelet (JBranehiostomd). 



a, mouth ; b, abdominal poms ; c, vent ; d, anterior end of notochord. 



found at almost every suitable locality within the temperate and 

 tropical zones. Their small size (rarely exceeding three inches), 

 transparency, and the rapidity with which they are able to bury 

 themselves into the sand are the causes why they so readily escape 

 observation, even at localities where they are known to be common. 

 Shallow, sandy parts of the coast, at some distance from the influx 

 of fresh water, seem to be the places on which the Lancelet may be 

 looked for. 



