No. 3-] DEVELOPMEXT OF MARINE SPONGES. 331 



very short and slender, and pointed at both ends (oxeate micros- 

 cleres). 



Structure of the Szvimming Larva. — The larva, when it 

 escapes from the body of the mother, is solid, of an oval shape, 

 with one unpigmented unciliated pole, the rest of the body 

 being covered with cilia and of a bright red color. It moves 

 rapidly about in the water, occasionally creeping, but usually 

 swimming, and it seems especially fond of making series of 

 long shallow dives, coming up to or near the surface between 

 the dives. The swimming larva can also change its shape to a 

 slight extent. 



The general ectoderm of the larva is composed of very long 

 and slender cells, cc., PI. XXI, Fig. ■]% (section through the 

 unpigmented pole of a larva just born), which contain the bright 

 red pigment. Each of these cells has a single flagellum, and 

 the nuclei contained in their inner ends make a broad, deeply 

 staining zone. The extreme peripheral ends of the cells are 

 modified to form a cuticle, cii. in Fig. 78. The columnar ecto- 

 derm cells become shorter towards the unpigmented pole, as is 

 shown in the figure, and yet pass with considerable abruptness 

 into the flat cells covering this pole. The latter cells do not 

 contain pigment, but are granular and stain deeply. The ecto- 

 derm remains unchanged during the free larval life (comp. PL 

 XXI, Fig. 81, longitudinal section through a swimming larva a 

 day old). 



Like the ectoderm, the parenchyma of the swimming larva 

 remains essentially the same throughout larval life — compare 

 the two sections. Fig. "jZ (through larva just born) and Fig. 81 

 (larva a day old). The parenchyma of the larva is much more 

 differentiated than it was in the stage shown in Fig. TJ. The 

 parenchyma cells of the latter stage were essentially alike and 

 were pretty evenly distributed, but during the last period of 

 embryonic life, they become variously modified. Some of them 

 crowd into the unpigmented end of the larva, becoming more 

 and more tightly packed, and forming ultimately a dense mass 

 of closely appressed polygonal cells, which stain faintly and 

 the cell outlines of which are distinguished with difficulty, /. c.. 

 Fig. 78. (In regard to this mass of cells, as in so many other 



