The nervous system of Brachiopods is not very clearly understood, and there are considerable discrepancies in the accounts of the various investigators, even when they are dealing with the same species. So much, however, seems certain, that there is a nervous ring surrounding the oesophagus, that this ring is enlarged dorsally, or, in other words, near the base of the lip, into a small and inconspicuous dorsal ganglion, and again ventrally or just behind the base of the tentacles into a ventral or sub-oesophageal ganglion. The latter is, contrary to what is usual in Invertebrates, of much larger size than the supra-oesophageal ganglion, but like the last named, it has retained its primitive connexion with the ectoderm or outermost layer of the skin. Both ganglia give off a nerve on each side which runs to the arms and along the base of the tentacles and lips. The sub-oesophageal ganglion also gives off nerves which supply the dorsal and ventral folds of the mantle, the muscles, and other parts.
The modified epithelium in connexion with the ganglia may possibly have some olfactory or tactile function, but beyond this the Brachiopoda would appear to be devoid of eyes, ears, or any other kind of sense organs,—a condition of things doubtless correlated with their sessile habits, and with the presence of a bivalved shell which leaves no part of their body exposed.
The Reproductive System
The majority of Brachiopods are bisexual, and many authorities regard the separation of sex as characteristic of the group; on the other hand, Lingula pyramidata is stated to be hermaphrodite, and it is not impossible that other species are in the same condition.
The generative organs are of the typical sort, that is, they are formed from modified mesoblastic cells lining the body cavity. These cells are heaped up, usually in four places, and form the four ovaries or testes as the case may be (Fig. [314]). The generative glands usually lie partly in the general body cavity and partly in the dorsal and ventral mantle folds, two on each side of the body. Along the axis of the heaped-up cells runs a blood-vessel, which doubtless serves to nourish the gland, the outer surface of which is bathed in the perivisceral fluid. Every gradation can be found between the ripe generative cell and the ordinary cell lining the body cavity. When the ova and spermatozoa are ripe they fall off from the ovary and testis respectively into the body cavity, thence they are conveyed to the exterior through the nephridia. The ova in certain genera, such as Argiope, Cistella, and Thecidium, develop in brood-pouches which are either lateral or median involutions of the body wall in the neighbourhood of the external opening of the nephridia; they are probably fertilised there by spermatozoa carried from other individuals in the stream of water which flows into the shell. In other species the ova are thrown out into the open sea, and their chances of meeting with a spermatozoon is much increased by the gregarious habits of their sessile parents, for as a rule considerable numbers of a given species are found in the same locality.
The Embryology
We owe what little we know of the Embryology of the group chiefly to Kowalevsky,[424] Lacaze-Duthiers,[425] and Morse.[426] The Russian naturalist worked on Cistella (Argiope) neapolitana, the French on Thecidium, and the American chiefly on Terebratulina.
Although this is not known with any certainty, it seems probable that the eggs of Brachiopods are fertilised after they have been laid, and not whilst in the body of the mother. The spermatozoa are doubtless cast out into the sea by the male, and carried to the female by the currents set up by the cilia clothing the tentacles.
In Thecidium, Cistella, and Argiope the first stages of development, up to the completion of the larva, take place in brood-pouches; in Terebratulina the eggs pass out of the shell of the mother and hang in spermaceti-white clusters from her setae and on surrounding objects. In the course of a few hours they become ciliated and swim about freely. The brood-pouch in Thecidium is median, in the convex lower shell, in Cistella it is paired, and arises by the pushing in of the lateral walls of the body in the region just behind the horse-shoe-shaped tentacular arms; the renal ducts, which also serve as oviducts, open into these lateral recesses.
In the female Thecidium (Fig. [317]) the two median tentacles which lie just behind the mouth are enlarged and their ends somewhat swollen; they are bent back into the brood-pouch, and to them the numerous larvae are attached by a short filament inserted into the second of the four segments into which the larva is divided. In Cistella a similar filament attaches the larvae to the walls of the brood-sac; thus they are secured from being washed away by the currents constantly flowing through the mantle cavity of the mother.