THE DEVELOPMENl' OK APLYSIA rUNCTATA. 



521 



The fibres are by that time fairly numerous, each one con- 

 sisting of a single spindle-shaped cell, showing longitudinal 

 striations. They pass dorsally to the left liver and are distri- 

 buted to the velum and. foot, and some are attached to the 

 oesophagus near the mouth. The smaller retractor muscle 

 appears later in development and always consists of three or 

 four fibres only. They are similar to those of the larger 

 muscle, and are no doubt of the satue origin. This muscle is 

 attached posteriorly to the dorsal body-wall on the right side, 



Text-fig. 10. - 



u.i. 



K.2 



St 



Similar view of a free-swimming larva. C.y. Cerebral ganglion. 

 P.g. Pedal ganglion. Other lettering as in Text-figs. 8 and 9. 



and thence appears to pass into the velum. As Mazzarelli 

 has noticed, there are no other muscu.lar elements of any 

 kind to be seen in the larva. 



The change in position of the large retractor muscle 

 noticed above is to be attributed to the fact that the left side 

 of the embryo, or more correctl}^, a particular zone in the left 

 side, grows more quickly than the corresponding zone on the 

 right side. This excess of growth on one side is a familiar 

 feature in discussions of the question of torsion. It was first 

 brought to notice by Biitschli. There is little evidence of its 



