cells are minute, and this has
especially stood in the way of embryological research. The plexus of
nerve-fibrils which underlie the ectoderm and are in places gathered
up into nerves, and the great development of connective tissue, are
worthy of notice. Much of the latter takes the form of hyaline
supporting tissue, embedded in which are scattered cells and fibres.
The lophophore and stalk are largely composed of this tissue. The
ectodermal cells are large, ciliated, and amongst the ciliated cells
glandular cells are scattered. The chitinous chaetae have their origin
in special ectodermal pits, at the base of which is one large cell
which is thought to secrete the chaeta, as in Chaetopods. These pits
are not isolated, but are connected by an ectodermal ridge, which
grows in at the margin of the mantle and forms a continuous band
somewhat resembling the ectodermal primordium of vertebrate teeth.

The ovary and testes are heaped-up masses of red or yellow cells due
to a proliferation of the cells lining the coelom. There are four of
such masses, two dorsal and two ventral, and as a rule they extend
between the outer and inner layer of the mantle lining the shells. The
ova and the spermatozoa dehisce into the body cavity and pass to the
exterior through the nephridia. Fertilization takes place outside the
body, and in some species the early stages of development take place
in a brood-pouch which is essentially a more or less deep depression
of the body-wall median in _Thecidea_, while in _Cistella_ (_?
Argiope_) there is one such pouch on each side, just below the base of
the arms, and into these the nephridia open. The developing ova are
attached by little stalks to the walls of these pouches. In spite of
some assertions to the contrary, all the Brachiopods which have been
carefully investigated have been found to be male or female.
Hermaphrodite forms are unknown.

[FIG. 29.--Three larvae stages of _Megathyris_ (_Argiope_). A, Larva
which has just left brood-pouch; B, longitudinal section through a
somewhat later stage; C, the fully formed embryo just before
fixing--the neo-embryo of Beecher. Highly magnified.

1. Anterior segment.
2. Second or mantle-forming segment.
3. Third or stalk-forming segment.
4. Eye-spots.
5. Setae.
6. Nerve mass (?).
7. Alimentary canal.
8. Muscles.]

_Embryology._--With the exception of