ECHINUS. 319 
When sexually mature the male gonads are of a milk-white 
or creamy hue, while the ovaries are yellowish or orange- 
brown in colour. 
OUTLINE OF LIFE-HISTORY. 
During the breeding season the spermatozoa and ova 
are discharged into the sea, where fertilisation of the ova 
takes place. Segmentation of the egg is total, though not 
quite regular, and results in the formation of a spherical 
blastula (Pl. V., fig. 38), consisting of a single layer of cells, 
enclosing a large segmentation cavity filled with fluid 
(seg. cav.). Hach cell bears a single flagellum on its outer 
surface, by the vibration of which the blastula revolves. 
Following immediately upon the completion of segmen- 
tation, a slight depression makes its appearance at one 
pole (Pl. V., fig. 39), and, at the same time, the cells 
forming the depression begin to bud actively. The 
resultant cells (mes.c.) are amoeboid; and, making their 
way all over the segmentation cavity or blastoccel, even- 
tually form the muscular and connective tissue systems of 
the adult. The depression deepens, and forms the 
archenteron or primitive intestine of the larva (Pl. V., 
fig. 43, arch.). The blastula has now become a gastrula. 
The dorsal surface of the gastrula is convex; its ventral 
surface concave; and it is bilaterally symmetrical. 
The blastopore (blas.) gradually moves forward on to the 
ventral surface, and the larva enters upon the “ Pluteus”’ 
stage (fig. 42). At the same time, the first skeletal 
structures appear in the shape of two lateral calcareous 
spicules (fig. 42, sk.7.), which support the body of the 
larva and the posterior ventral arms (p.ven.a.), which 
begin, at this stage, to grow out from the ventral surface. 
The anterior blind end of the archenteric tube now deve- 
lops, on each side, a small out-growth (fig. 42), which 
