Lamp shells were common at the beginning of the Cambrian period some six hundred million years ago. Their shells are readily fossilised so there is a good record of them from this time on. They were very common in ancient seas but only a few forms have survived to modern times. In the past there were 30,000 species but most died out in the great Permian extinction and only about 300 are found today. Superficially lamp shells look like clams, but they are different enough to deserve their own phylum and in fact are much more closely related to ectoprocts and phoronids, having a true coelom and a circle of tentacles called a lophophore on their heads.
The bivalved shell is secreted by a layer of skin called a mantle much as in molluscs. The animals lead sessile lives as adults, attaching themselves to a substrate by a pedicell and then filter feeding with the lophophore for a living. Brachiopods traditionally have been divided into two groups: the Inarticulata, which have no hinge between the shells; and the Articulata which have a hinge. The shells of the inarticulates are held together by muscles only. They burrow in sand or mud and pull their bodies out of danger by contracting their pedicel. Other inarticulates attach themselves permantently to rocks by the ventral valve of their shell.
There are other differences between the groups as well. The inarticulate nervous system is in the epidermis while the articulate nervous system is below. During embryological development, cleavage is radial in the inarticulates and modified in the articulates. Later development differs greatly between the two groups also. For this reason many taxonomists have given up on the terms inarticulata and articulata and have subdivided the brachiopods by other characteristics than the hinges on their shells. It may be that the group is polyphyletic and that different species arose separately from phoronid-like ancestors. Genetic analysis may eventually shed light on this subject.
Brachiopods have been quite large in the past, with shells a third of a meter across having been found in the fossil record. Modern lamp shells do not get quite so big. They have no respiratory organs but they do have blood, containing the respiratory pigment, haemerythrin. The blood moves through an open circulatory system and there is a main pulsating vessel dorsally to pump the blood. Circulation is sluggish but must be enough for these sessile animals.
Lamp shells are deuterostomes. The blastopore closes and another opening becomes the mouth. They are probably the most primitive animals in the deuterostome line and so are of interest from an evolutionary perspective.
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press. http://www.ucmp.berkeley.edu/brachiopoda/brachiopoda.html
