this motion. In most of the animals, and also in many of the lower
plants (but not the higher) each of these spermatozoa has a very
small, naked cell-body, enclosing an elongated nucleus, and a long
thread hanging from it (Figure 1.20). It was long before we could
recognise that these structures are simple cells. They were formerly
held to be special organisms, and were called "seed animals"
(spermato-zoa, or spermato-zoidia); they are now scientifically known
as spermia or spermidia, or as spermatosomata (seed-bodies) or
spermatofila (seed threads). It took a good deal of comparative
research to convince us that each of these spermatozoa is really a
simple cell. They have the same shape as in many other vertebrates and
most of the invertebrates. However, in many of the lower animals they
have quite a different shape. Thus, for instance, in the craw fish
they are large round cells, without any movement, equipped with stiff
outgrowths like bristles (Figure 1.21 f). They have also a peculiar
form in some of the worms, such as the thread-worms (filaria); in this
case they are sometimes amoeboid and like very small ova (Figure 1.21
c to e). But in most of the lower animals (such as the sponges and
polyps) they have the same pine-cone shape as in man and the other
animals (Figure 1.21 a, h).

When the Dutch naturalist Leeuwenhoek discovered these thread-like
lively particles in 1677 in the male sperm, it was generally believed
that they were special, independent, tiny animalcules, like the
infusoria, and that the whole mature organism existed already, with
all its parts, but very small and packed together, in each
spermatozoon (see Chapter 1.2). We now know that the mobile
spermatozoa are nothing but simple and real cells, of the kind that we
call "ciliated" (equipped with lashes, or cilia). In the previous
illustrations we have distinguished in the spermatozoon a head, trunk,