STRUCTURE OF THE LARVA OF SPONGILLA LACUSTRIS. 383 



are connected by transitional stages^ but which may never- 

 theless be described apart from one another. The flagellated 

 cells are alike in all the types, and need no further discussion. 

 The only diff'erence that exists between their early and late 

 condition is that their nuclei are nearly on the same level, and 

 consequently that they are almost the same length. In addi- 

 tion to this, in the older larvae the cells with granular nuclei 

 are seen to push their processes between the flagellated cells, 

 indicating that fixation and metamorphosis cannot be long 

 delayed. The differences which exist between the several 

 types of larvae to be considered concern the inner mass, 

 and the special features of each type will be described in the 

 following pages. 



Special Features of Type A. 



The inner mass in this type consists of only two kinds of 

 cells, namely, cells with granular nuclei (a) and with vesicular 

 nuclei (j3). 



The former are almost entirely confined to two positions, 

 occurring first as an incompletely developed layer of cells 

 immediately below the flagellated layer; and secondly, in the 

 vicinity of the larval cavity, where, however, they are almost 

 completely limited to its anterior border. They have not yet 

 assumed the flattened shape which they acquire in later stages in 

 either of these positions. 



The latter have the monopoly of the interior of the solid 

 posterior end of the inner mass. They enclose in many cases 

 three or four nutritive vacuoles, together with numerous yolk 

 bodies, and occasionally a developing spicule is found in them. 



Microscleres and cell groups are not found in this type, 

 which is evidently the youngest of all free-swimming laivaj 

 (fig. 1). From it all the others are developed, and it is highly 

 probable that the larva never fixes while in this early state of 

 differentiation of the inner mass, but proceeds to develop into 

 one or other of the types which remain to be described. 



