STRUCTURE OP THE LARVA OF SPONGILLA LACUSTRIS. 395 



metamorphosis. Type A seems to be incapable of fixation, 

 owing to the fact that the cells with granular nuclei are as yet 

 few in number. The immediate result of the fixation of types 

 B and C is absolutely different from that obtained when type 

 D settles down to its sessile life. When a larva fixes after 

 attaining the stage of development described above in type C, 

 flagellated chambers which have acquired their definitive 

 structure are present as well as the rudiments of the exhalant 

 system. But when a larva possessing the structure described 

 in type D becomes fixed, both the flagellated chambers and the 

 rudiments of the exhalant system are almost completely absent. 



The Actual Fixation of the Larva. — The larva fixes 

 either by the anterior pole or by a point on the side not further 

 back than the line of separation of the larval cavity from the solid 

 posterior part of the inner mass. No larva was observed to fix 

 by the posterior pole, or by a point near it. The fixation is 

 brought about by the passing out of the cells with granular 

 nuclei that lie beneath the flagellated cells. The larva just 

 before it settles down turns about much in the same way as a 

 spinning-top does when about to end its spin. The point at 

 which the cells with granular nuclei make their way out 

 corresponds to the peg of the spinning-top in the above 

 comparison. That point in the flagellated layer seems to be, 

 as it were, paralysed. At the time when the cells with granular 

 nuclei have actually penetrated the flagellated layers, and are 

 beginning to spread themselves along the surface of fixation, 

 the motion of the larva as a whole inevitably ceases, though 

 the flagella of the flagellated cells situated elsewhere may 

 continue in motion for some length of time. 



The Obliteration of the Larval Cavity after Fixa- 

 tion. — The persistence or obliteration of the larval cavity 

 depends upon the structure of the larva. When type D fixes 

 itself by the anterior end, the solid part of the inner mass, which 

 lies at the posterior end, approaches the surface of fixation, and 

 the larval cavity is thereby reduced to mere slits (fig. 15), 

 and ultimately disappears completely (fig. 16), the fixed larva 

 flattening out fairly symmetrically in all directions. But when 



VOL. 42, PART 4. NEW SERIES. E B 



