Yatsu's article on the
development of _Lingula_ (_J. Coll. Sci., Japan_, xvii., 1901-1903)
and E.G. Conklin's on "Terebratulina septentrionalis" (_P. Amer. Phil.
Soc._ xli., 1902), little real advance has been made in our knowledge
of the embryology of the Brachiopoda within recent years. Kovalevsky's
researches (Izv. Obshch. Moskov, xiv., 1874) on _Megathyris_
(_Argiope_) and Yatsu's just mentioned are the most complete as
regards the earlier stages. Segmentation is complete, a gastrula is
formed, the blastopore closes, the archenteron gives off two coelomic
sacs which, as far as is known, are unaffected by the superficial
segmentation of the body that divides the larva into three segments.
The walls of these sacs give rise at an early stage to muscles which
enable the parts of the larva to move actively on one another (fig.
29, B). About this stage the larvae leave the brood-pouch, which is a
lateral or median cavity in the body of the female, and lead a free
swimming life in the ocean. The anterior segment broadens and becomes
umbrella-shaped; it has a powerful row of cilia round the rim and
smaller cilia on the general surface. By the aid of these cilia the
larva swims actively, but owing to its minute size it covers very
little distance, and this probably accounts for the fact that where
brachiopods occur there are, as a rule, a good many in one spot. The
head bears four eye-spots, and it is continually testing the ground
(fig. 29, A, C). The second segment grows downwards like a skirt
surrounding the third segment, which is destined to form the stalk. It
bears at its rim four bundles of very pronounced chaetae. After a
certain time the larva fixes itself by its stalk to some stone or
rock, and the skirt-like second segment turns forward over the head
and forms the mantle. What goes on within the mantle is unknown, but
presumably the head is absorbed. The chaetae drop off, and the
lophophore is believed to arise from thickenings which appear in the
dorsal mantle lobe. The Plankton Expedition brought back, and H.
Simroth (_Ergeb. Plankton Expedition_, ii., 1897) has described, a few
larval brachiopods of undetermined genera, two of which at least were
pelagic, or at any rate taken far from the coast. These larvae, which
resemble those described by Fritz Muller (_Arch. Naturg._, 1861-1862),
have their mantle turned over their head and the larval shel