and a rudimentary auditory vesicle. The nose is a single pit, as in
the Amphioxus. The two sections of the gut are also just the same and
very rudimentary in the lamprey. On the other hand, we see a great
advance in the structure of the heart, which is found underneath the
gills in the shape of a centralised muscular tube, and is divided into
an auricle and a ventricle. Later on the lamprey advances still
further, and gets a skull, five cerebral vesicles, a series of
independent gill-pouches, etc. This makes all the more interesting the
striking resemblance of its immature larva to the developed and
sexually mature Amphioxus.

While the Amphioxus is thus connected through the Cyclostoma with the
fishes, and so with the series of the higher vertebrates, it is, on
the other hand, very closely related to a lowly invertebrate marine
animal, from which it seems to be entirely remote at first glance.
This remarkable animal is the sea-squirt or Ascidia, which was
formerly thought to be closely related to the mussel, and so classed
in the molluscs. But since the remarkable embryology of these animals
was discovered in 1866, there can be no question that they have
nothing to do with the molluscs. To the great astonishment of
zoologists, they were found, in their whole individual development, to
be closely related to the vertebrates. When fully developed the
Ascidiae are shapeless lumps that would not, at first sight, be taken
for animals at all. The oval body, frequently studded with knobs or
uneven and lumpy, in which we can discover no special external organs,
is attached at one end to marine plants, rocks, or the floor of the
sea. Many species look like potatoes, others like melon-cacti, others
like prunes. Many of the Ascidiae form transparent crusts or deposits
on stones and marine plants. Some of the larger species are eaten like
oysters. Fishermen, who know them very well, think they are not