STRUCTURE AND LIFE-HISTORY OF A SPONGE. 
29 
singular thing that the male elements have not yet been discovered 
in Sycandra raphanus, but they have been seen in various other 
sponges ; in some species occurring in the same individuals as 
contain the ova, in others in different individuals, so that some 
sponges are hermaphrodite, and others bisexual. 
If the ova and spermatazoa be kept separate from each other, 
nothing remarkable will happen, each will lead its own life, and 
die a natural death. But they do not as a rule remain apart ; the 
spermatazoa when set free on the disruption of the sperm balls, 
are carried away in the out-flowing water from the radial tubes of 
one individual, to enter along with the incurrent water the radial 
tubes of another. Should this latter contain ova, the spermatazoa 
on approaching an ovum swim towards it like so many tadpoles, 
and striking it head foremost, with their tails streaming outwards, 
remain for a while radiately disposed about its circumference. 
Finally they enter the substance of the ovum, are absorbed and 
disappear. The ovum is then said to be impregnated, Unfor- 
tunately this process, though it has been witnessed in many other 
sponges, has not been observed in Sycandra raphanus. As a 
consequence of the fusion of the spermatazoa with an ovum, the 
latter undergoes a remarkable series of changes, which end in pro- 
ducing a fresh sponge like the parent. First of all the nucleus 
disappears, and after awhile two fresh nuclei appear in its place ; 
a constriction then occurs round the exterior of the ovum, as a 
shallow furrow, which deepens, extending inwards between the two 
nuclei till it actually divides the ovum into two parts, each of 
which has the value of a true cell. The direction in which this 
division takes place is constantly from the top of the ovum (that 
side facing the endodermal layer), to the bottom, or as we may 
briefly formulate it — perpendicular. The cells resulting from the 
division are flattened below and also against each other, but 
rounded off towards the top, so that they may be said to diminish 
in size from the base upwards. They soon undergo the same kind 
