LANCELETS. 



289 



the child does not resemble the parents, but the grand- 

 parents, another example of alternation of 

 generations. 



The Tunicates are all marine, and they 

 abound in the seas of all parts of the world. 

 Some of them are known from their shapes 

 and color as 'sea-peaches/ others as 'sea- 

 pears/ while a common name for all is 'sea- 

 squirts/ due to the fact that they squirt water from the 

 openings upon being disturbed. 



manhaitensis ^ 



BRANCH II. LEPTOCARDII (LANCE LETS). 



The few species of lancelets (Amphioxus} are all marine 

 and occur in warmer seas. They have a body which is 

 fish-like, but thev differ from all fishes in the absence of a 



/ 



true heart and of a skull. The gill-slits are numerous 



FIG. 100. Diagram of Amphioxus (after Hertwig and Boveri). Above (dotted) 

 is the nervous system; below it (cross-lined), the notochord; the mouth is 

 surrounded by the circle of tentacles; below the notochord is the region of 

 gill-slits; the vent is near the posterior (right) end below. 



(about sixty), and these empty into a gill-chamber recall- 

 ing in some features that of the tadpoles. The notochord 

 runs the whole length of the body, and a stomach is lack- 

 ing, the liver emptying into the intestine just behind the 

 gills. Limbs or paired fins are absent, but there is a 

 median fin at the end of the body. The animals are about 

 two or three inches long, are almost perfectly transparent, 

 and bury themselves in the sand, only the mouth end, 



