No. 3.] DEVELOPMENT OF MARINE SPONGES. 231 
very short and slender, and pointed at both ends (oxeate micros- 
cleres). 
Structure of the Swimming 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, ec., Pl. XXI, Fig. 78 (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, cv. 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. 78 (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. 77. 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 
