key concept 31.1 Protostomes Account for More Than Half of All Described Species

659

You may recall that the embryos of diploblastic animals (the ctenophores, placozoans, and cnidarians, which we discussed in Chapter 30) have two cell layers: an outer ectoderm and an inner endoderm (see Key Concept 30.1). Sometime after the origin of the diploblastic animals, a third embryonic cell layer evolved: the mesoderm, which lies between the ectoderm and the endoderm. Mesoderm is found in the two major triploblastic animal clades, the protostomes and the deuterostomes. If we were to judge solely on the basis of numbers, both of species and of individuals, the protostomes would emerge as by far the more successful of the two groups.

focus your learning

  • Ecdysozoans have an external covering—a cuticle—that they must shed as they grow.

  • Arthropods have a rigid exoskeleton and have made use of a great variety of appendages.

As noted in Key Concept 30.1, the name “protostome” means “mouth first.” In protostomes, the embryonic blastopore becomes the mouth as the animal develops. In contrast, in deuterostomes (“mouth second”), the blastopore becomes the anal opening of the gut. The protostomes are extremely varied, but they are all bilaterally symmetrical animals whose bodies exhibit two major derived traits:

  1. An anterior brain that surrounds the entrance to the digestive tract

  2. A ventral nervous system consisting of paired or fused longitudinal nerve cords

Other aspects of protostome body organization differ widely from group to group (Table 31.1). Before gene sequences were available for phylogenetic analysis, biologists considered the structure of the body cavity to be a critical feature in animal classification. But the results of genetic analyses have shown that body cavity forms have undergone considerable convergence in the course of protostome evolution. Although the common ancestor of the protostomes had a coelom, subsequent modifications of the coelom distinguish many protostome lineages. In some lineages (such as the flatworms and entoprocts), the coelom has been lost (that is, these groups reverted to an acoelomate state). Some lineages are characterized by a pseudocoel, a body cavity lined with mesoderm in which the internal organs are suspended (see Figure 30.6B). In two of the most prominent protostome clades, the coelom has been highly modified:

table 31.1 Anatomical Characteristics of Some Major Protostome Groups
Group Body cavity Digestive tract Circulatory system
Arrow worms Coelom Complete None
Lophotrochozoans
Bryozoans Coelom Complete None
Entoprocts None Complete None
Flatworms None Blind gut None
Rotifers Pseudocoel Complete None
Gastrotrichs Pseudocoel Complete None
Ribbon worms Coelom Complete Closed
Brachiopods Coelom Complete in most Open
Phoronids Coelom Complete Closed
Annelids Coelom Complete Closed or open
Mollusks Reduced coelom Complete Open except in cephalopods
Ecdysozoans
Nematodes Pseudocoel Complete None
Horsehair worms Pseudocoel Greatly reduced None
Arthropods Hemocoel Complete Open

The protostomes can be divided into two major clades—the lophotrochozoans and the ecdysozoans—largely on the basis of DNA sequence analysis (Figure 31.1).

Activity 31.1 Features of the Protostomes

www.life11e.com/ac31.1

image
Figure 31.1 Phylogenetic Tree of Protostomes Two major lineages, the lophotrochozoans and the ecdysozoans, dominate the protostome tree. Some small groups are not included in this tree. The phylogenetic relationships shown here are supported mainly by genomic sequence data. Although genomic studies are contributing greatly to our knowledge of animal phylogeny, most species of protostomes have yet to be studied in detail.

Activity 31.2 Protostome Classification

www.life11e.com/ac31.2