ZOOGENESIS 



mals are the actinotrocha larvas (fig. 102., p. 175) of 

 the phoronids, and the cyphonautes larvae (fig. 90, 

 p. 175) of most polyzoans. There are various other 

 larval types among the marine animals. 



Not only do the different types of larvae vary very 

 widely in their body form, but they also vary widely 

 in the manner of their development. In most crusta- 

 ceans, whatever may be the sequence of their larval 

 forms, the development is more or less direct. In the 

 nemerteans the development is usually direct, but in 

 some there is a curious larva called a pilidium (fig. 106, 

 p. 175) which transforms to the adult by a very com- 

 plicated metamorphosis. There are also other forms 

 of larvae in the nemerteans. 



In some of the trematodes the development is 

 direct, but in others it is extremely complicated. 

 Thus the first larva may be a miracidium (fig. 96, 

 p. 175) which transforms into a sporocyst (fig. 98, 

 p. 175), or rarely into a redia (fig. 97, p. 175). The 

 sporocyst may produce other sporocysts, but usually 

 produces rediae, these last — sometimes also the sporo- 

 cysts — producing curious tadpole-like larvae called 

 cercarias (fig. 99, p. 175), which grow into the adults. 



During the course of their development the echino- 

 derms become transformed from a bilateral larva (figs. 

 88, 89, p. 175) into a radially symmetrical adult (figs. 

 41, 4^, p. 71) with the body in five divisions. 



In the passage from the larval to the adult form 

 quite a number of the attached sea animals turn a half- 

 somersault. The larva attaches itself at the anterior 

 end just in front of the mouth and fastens itself 



