Station 1: Introduction to the Lophotrochozoans

Before proceeding with this lab you should review the textbook and lecture material on this subject. In particular you should be able to define the following terms:

• Blastopore

• Coelom

• Ectoderm

• Endoderm

• Gastrula

• Lophophore

• Lophotrochozoa

• Mesoderm

• Protostome

• Trochophore Larva

You should also be able to describe the following concepts in your own words:

• How the Lophotrochozoans are developmentally different than the Cnidarians

In the previous lab, you saw how body plans become more complex in animals that have tissues, bilateral symmetry, organs, and triploblastic embryological development. These four ancestral traits are shared by animals in most animal phyla. In this lab topic we will look at bilaterally-symmetrical animals in detail, illustrating how these bilateral animals areg rouped into nested clades based on shared derived traits.

Phylogeny of animalsClick for more options Bilaterally symmetrical-animals are divided into two clades based on molecular similarities that correlate with embryological characteristics (see figure). Understanding the gastrula stage of embryonic development and the development of the digestive tube in bilaterally-symmetrical animals is the key to understanding the basis for these clades. 

In one clade, the protostomes, the initial opening (blastopore) of the digestive tube in the gastrula will become the mouth in the adult animal. In the other clade, the deuterostomes, the blastopore becomes the anus. In either case, the development of the digestive tube establishes the anterior-posterior axis. 

Correlated with the fate of the blastopore in these two groups is a second embryological characteristic: development of the mesoderm, one of three embryonic tissues called germ layers. Consequently, bilaterally-symmetrical animals are said to be triploblastic at the late gastrula stage. All other cells found in the adult animal arise from these layers, but not in a random pattern. They follow, strict lines of development. The outer layer of the gastrula, the ectoderm, will form the outer covering of the animal and the nervous system. The cells lining the digestive tube of the gastrula, the endoderm, develop into the lining of the digestive system and digestive glands. The remainder of the adult animal's body (muscles, skeleton, connective tissues, reproductive tissues,etc.) comes from the third layer of cells, the mesoderm. 

In addition, most bilaterally-symmetrical animals have a tube-within-a-tube body plan with a body cavity (coelom). You have seen a coelom if you have ever cleaned a fish or a turkey before cooking. It is the cavity in which the organs are found. A coelom has multiple functions: organs contained in it can enlarge or move independent of the body wall, such as the stomach expanding while feeding and later emptying; and fluids in the cavity can cushion internal organs from injury, serve as a primitive circulatory system, or act as a hydrostatic skeleton. 

The embryological origin of the coelom differs between the protostome and the deuterostome clades.  

Genetic analysis supports the protostome/deuterostome dichotomy among the bilaterally-symmetrical animals. About four phyla are considered deuterostomes - the rest, some 30 phyla, are protostomes. The molecular data indicate protostomes include two clades, a group called the lophotrochozoans and another called the ecdysozoans. 

In this lab you will look at some representative lophotrochozoans and in the next lab you will study the ecdysozoan phyla. Although the lophotrochozoan clade is based on genetic similarities, it is a diverse group. The name, lophotrochozoan is a made-up word, a recent addition to the language of biology. It attempts to capture two morphological characteristics seen among members of the clade. Not all of the included phyla have both characteristics. About four have a horseshoe-shaped feeding structure called a lophophore which consists of ciliated tentacles. Several, but not all, of the other phyla have a larval stage in their life cycle called a trochophore. Some phyla in the clade have neither a lophophore nor a trochophore, but are included because of genetic and molecular similarities.

Clade Lophotrochozoa

Lophotrochozoa is a clade of protostome animals. The taxon was established as a monophyletic group based on molecular evidence.