A further step was made in 1860 and 1861 by the discovery and description of the larvae of Brachiopoda, by F. Müller and Lacaze-Duthiers. Since that time we owe what little advance has been made in the embryology of the group to the researches of Morse and of Kowalevsky.
Modern methods of research—section cutting, etc.—were first applied to the group by the Dutch naturalist, van Bemmelen,[413] from whose admirable historical account of our knowledge of the group many of the above facts have been gathered. These methods have thrown considerable light upon the histology of the group, but have not added very much to our knowledge of the structure or the affinities of the Brachiopoda. The modern views as to the latter point may be best discussed after some account of the anatomy of the various genera has been given.
The Shell
The body of a Brachiopod is enclosed within a bivalve shell, but the two halves are not, as they are in the Pelecypoda, one on each side of the body, but occupy a different position with regard to the main axes of the body. What this position is, has formed the subject of a good deal of discussion. For our purpose, however, it will suffice to distinguish the two valves by the most commonly accepted terms of dorsal and ventral. The former is, as a rule, the smaller of the two, and usually lies on the lower surface of the animal in life. Adopting the orientation indicated above, the stalk by means of which the Brachiopoda are attached to the rocks and stones, etc., upon which they live, becomes posterior, and the broader edge of the two shells, which are capable of being opened to some extent, is anterior.
Fig. 312.—Four specimens of Terebratulina caput serpentis, attached to a waterlogged piece of wood, from the Clyde area.
The posterior end of the shell usually narrows, and the ventral valve projects behind the dorsal, and may be produced into a sort of beak or funnel, through the aperture of which the stalk protrudes. This aperture may be completed by the ventral shell, or the latter may only be notched, in which case the hole is completed by the posterior edge of the dorsal shell.
The nature of the shell has been used in classifying the group into two orders:—
I. The Ecardines, whose shell is chitinous but slightly strengthened by a deposit of calcareous salts. There is no hinge and no internal supports for the arms. The alimentary canal terminates in an anus.
II. The Testicardines, whose shells are composed of calcareous spicules. The valves are hinged together, and there is usually an internal skeleton supporting the arms. There is no anus.