THE DEVELOPMENT OF APLYSIA PUNCTATA. 511 



everted. It is at first very thin and transparent^ and even at 

 its fullest development in tlie free-swimming- larva never 

 becomes thick or resistant enough to interfere with section- 

 cutting. It grows at once into its ultimate exogastric form, 

 and is always perfectly symmetrical. In the free-swimming 

 larva it is marked by a number of fine lines, forming an 

 irregular network. 



The Anal Cells. — These have been already described {Ac. 

 in PI. 22). As development proceeds they decrease in size, 

 this reduction being probably correlated with the growth of 

 the secondary kidney, Avhich takes on the function of excre- 

 tion. In the free-swimming larva they are still prominent 

 features, though they are neither figured nor described in this 

 stage by Mazzarelli. They presumably disappear towards the 

 end of the larval period. 



The Otocysts. — These arise as ectodermal invaginations 

 of about six cells, one on eacb side of the rudiment of the foot 

 {ot. in PI. 22). Later some ten or twelve cells sink well below 

 the surface and form closed vesicles of some size, which are 

 very obvious in the living larva lying at the base of the foot, 

 below and to the sides of the oesophagus. At first these 

 vesicles seem to be empty, but towards the end of embryonic 

 life a large spherical otolith is very conspicuous inside each. 



The Nervous System. — We have seen no trace of the 

 nervous system before a stage corresponding to PI. 22, fig. 8. 

 In such an embryo there are visible rudiments of both cerebral 

 and pedal ganglia (<•.(/. and jj.^.). Our preparations do not 

 make the mode of origin of the nervous system very clear. It 

 would appear to arise as a cell-proliferation from the ecto- 

 derm, as there is no evidence of an ectodermal invagination 

 to form the ganglia, as occurs in some forms, Dentalium for 

 example. When the ganglia first appeal', they take the form 

 of slight thickenings in close contact with the ectoderm. 

 The cerebral ganglia lie just above the mouth, the pedal 

 ganglia to the outer and ventral sides of the otocysts, and 

 slightly anterior to them. The ganglia become more definite 

 and larger, but in the free-swimming larva they -are still near 



