THE EVOLUTION OF INVERTEBRATES 



XI. MOLLUSCA 



Class 1. Pelecypoda ; example, Mya arenaria. 

 Class 2. Gasteropoda ; example, Helix nebulosa. 

 Class 3. Cephalopoda; example, Nautilus pompilius. 



301 



PROBABLE ANCESTORS OF THE VERTEBRATES 



The Lancelet. It is always important to find, if possible, 

 connecting links between large and seemingly distinct groups 

 of animals. The words " invertebrate " and " vertebrate " 

 were once thought to be terms which together included all 

 animals ; but zoologists have discovered several animals 

 which are neither vertebrate nor invertebrate in the usual 

 sense of the words. 



The first of the three animals to be described in this sec- 

 tion of the chapter is a vertebrate of a very simple type. 



FIG. 148. The Lancelet. x 2. (After Kowalevsky) 



1, mouth; 2, gill-slits; 3, atrium; 4, atrial pore; 5, intestine; 6, anus; 7, noto- 

 chord; 8, nerve-cord 



The lancelet (Amphiox'us lanceola'tus, Fig. 148) is a fish-like 

 animal about two inches long. It lives almost imbedded in 

 the sand at the sea-bottom in Chesapeake Bay and in other 

 warm ocean waters. 



The mouth (Fig. 148, 1) of the lancelet opens into a long 

 pharynx, which has many pairs of gill-slits (Fig. 148, 2). 

 When water and food pass in at the mouth, the water passes 

 through the gill-slits, giving up oxygen to the blood in the 

 gills, and then passes into a chamber partially surrounding 

 the pharynx, called the atrium (Fig. 148, 3), and to the 



