Question

What are the layers to the shell of brachiopods? What secretes the first layer? What makes up the second layer in articulate and inarticulates?

Answer 1

Brachiopods have two layers in their shell; the periostracum as the first layer secreted by the mantle, and a second layer of calcium carbonate for articulate brachiopods or calcium phosphate for inarticulate brachiopods.

Step-by-step explanation:

The brachiopod shell has two layers and while they share some superficial resemblance to bivalves, their shell structure is quite unique. The first layer, known as the periostracum, is secreted by the mantel tissue. This outer organic layer provides initial protection and may deter predators or prevent fouling. The internal layers constitute the main shell and vary between two types of brachiopods: articulate and inarticulate brachiopods.

In articulate brachiopods, the second layer contains hard mineralized materials, typically calcium carbonate in two forms - calcite and aragonite. This composition contributes to the rigidity and protective capacity of the shell. In contrast, inarticulate brachiopods have a second layer that is mostly constituted of calcium phosphate along with some organic material, which makes it different in both texture and composition from that of their articulate counterparts.

Answer 2

The brachiopod shell has two main layers: the organic cuticle known as the periostracum and the calcareous layer beneath it, which differs in composition between articulate and inarticulate brachiopods.

Step-by-step explanation:

The shell of brachiopods consists of two main layers. The first layer, known as the periostracum, is an organic cuticle that is secreted by the outer mantle epidermis of the brachiopod. This layer serves as a protective cover for the subsequent calcareous layers.

The second layer varies between articulate and inarticulate brachiopods. In articulate brachiopods, the second layer is composed of hard calcium carbonate arranged in a specific lattice structure, which gives it a distinctive appearance and mechanical strength. In inarticulate brachiopods, this layer can be made up of calcium phosphate mixed with calcium carbonate or entirely of carbonate. It provides the main structural support for the shell.