Molluscoids, a division of the old branch of mollusca first made by Milne-Edwards to in-clude the bryozoa and ascidians or tunicates, to which have since been added the brachio-pods; all of which arc now regarded by Prof. E. S. Morse and others as articulates, having certain molluscan affinities, but coming nearest to the tubicolous worms. (See Brachiopoda, and Beyozoa.) In the lowest of these, the uryozoa, are comprised small pedunculated animals, the margin of whose body is provided with vibratile cilia, for producing the w^ater currents necessary to respiration and to the obtaining of food; these cilia are sometimes supported on long tentacular prolongations; the digestive cavity is distinct from the walls of the body, and can be traced as a canal from mouth to vent, both opening within the ciliated circle, being reflected upward; they propagate by buds and by free swimming ciliated gem-mules. They seem to have both males and females on the same stem, the cells containing animals with eggs being apparently more numerous than those with spermatozoa; the muscular system is largely developed, and serves principally to retract the animal within its cell.
They have been divided by Van der Hoeven into the families: 1, stelmatopoda, in which the tentacles are disposed in a zone around the mouth, as in the genera escliara, flustra, and cellularia; and 2, lophopoda, with tentacles set pectinately on two arms, and numerous, such as cristatella, plumatella, and alcyonella. The tunicata, including the ascidians and salpae, have no shells, but are enclosed by an elastic, cellulose, uncalcified integument, having two apertures; the circulation is peculiar in the phenomenon of venous blood at one time proceeding from the heart to the gills, and at another arterial blood from the gills to the heart, in the same vessels; respiration is effected either by a vascular ciliated pharyngeal sac, or by a ribbon-shaped gill stretched across the common visceral cavity; the nervous system presents a single ganglion, from which the nerves radiate; organs of feeling, sight, and even hearing, have been described in these animals; muscular fibres, both longitudinal and transverse, are well developed.
In salpce we have free swimming animals, drawing in water by one aperture and expelling it by another one opposite; they are numerous in the Mediterranean and in the temperate parts of the ocean far from the shores, and are said to be phosphorescent at night; they sometimes occur singly, and sometimes in long chains or in rings; Chamisso concluded, from observing the living animals, that a generation of distinct salprc alternates with one of those in a chain; within the single individuals connected embryos were found, which, with other similar phenomena, led to the interesting work of Steenstrup on the "Alternations of Generation;" the solitary salpae are sexless, and are propagated by internal germs or buds, and are inferior to the associated forms, which have reproductive organs; the latter produce each a single young one. The ascidians have a sac-like body, with two apertures generally near together; the branchial sac is large, the opening of the oesophagus situated at the bottom; they are mostly attached, and propagate both by eggs and buds, the male and female organs being on the same individual. They are both simple and compound.
Those young which originate from eggs move free in the early stage, and have a long tail which is lost when they fix themselves by the opposite extremity; in the compound forms, larvae of this description may enclose a group of eight united ascidians, by their division laying the foundation for a colony while yet free, capable of greater multiplication by further gemmation. Ascidians are found from the tropics even into the arctic regions, and some of the compound forms are brilliantly phosphorescent. The non-peduncu-lated single ascidians were known to Aristotle, and were called by him tethvon; sometimes called bagpipes, these animals are often seen attached to rocks, shells, crabs, and other bodies; though several may be found in a group, they do not form a compound body with a common external covering; they are occasionally found attached to a shapeless mass formed by the bodies of other ascidians. The food consists of small organic particles, which are brought with the water into the branchial sac and to the oesophageal opening at the bottom. - The brachiopods, or palliobranchiata, have the body depressed, covered with a mantle, bilobed and open; the branchiae are not separated from the mantle; the heart is double and arterial; near the mouth are two long spirally convoluted arms (whence the name of the order) provided with cirri or cilia; the mouth is simple, at the base of the arms; the shell is bivalve, always attached either by a peduncle or by the shell, and adheres to the mantle by several oblique muscles; there is no elastic ligament at the hinge, which is opened by the arms and by internal muscles; all are aquatic and marine.
They include the terebratulm; the extinct spirifers, or this and prodnctus, with articulated calcareous shells; and the crania and lingula, the last interesting as occurring with slightly modified species and with few interruptions from the Silurian to the present epoch.