Methodological guidelines on tasks
Arthropods are a vast assemblage of animals. At least three quarters of a million species have been described; this is more than three times the number of all other animal species combined (see figure inside cover). The tremendous adaptive diversity of arthropods has enabled them to survive in virtually every habitat; they are perhaps the most successful of all the invaders of the terrestrial habitat. Arthropods represent the culmination of evolutionary development in the protostomes. They arose either from primitive stocks of polychaetes or from ancestors common to both, and the relationship between arthropods and annelids is dis¬played in several ways.
- Arthropods, like annelids, are metameric. Metamerism is evident in the embry-onic development of all arthropods and is a conspicuous feature of many adults, especially the more primitive species. Within many arthropod groups there has been a tendency for metamerism to become reduced. In such forms as mites, for example, it has almost disappeared. Loss of metamerism has occurred in three ways. Segment become lost, segments have become fused together, and segmental structures, such as ц ages, have become structurally and functio. ferentiated from their counterparts on segments. Different structures having the same segmental origin arc said to be serially homologous. Thus, the second antennae of a crab are serially homologous to the chclipeds (claws), for evolved from originally similar segmental appendages.
- In the primitive condition each arthropod segment bears a pair of appendages. This same condition is displayed by the polychaetes, in which each metamere bears a parapodia. However, the homology between pi podia and arthropod appendages is uncertain.
- The nervous systems in both groups are constructed on the same basic plan. In both a dorsal anterior brain is followed by a ventral nerve cod containing ganglionic swellings in each segment.
- The embryonic development of a few pods still displays holoblastic determinate dermage, with the mesoderm in these forms arising from the 4d blastomere.
Although arthropods display these annelidan char¬acteristics, they have undergone a great many pro¬found and distinctive changes in the course of their evolution. The distinguishing feature of arthropods, and one to which many other changes are related, is the chitinous exoskeleton, or cuticle, that covers the entire body. Movement is made possible by the division of the cuticle into separate plates. Primitively, these plates are con¬fined to segments, and the plate of one segment is connected to the plate of the adjoining segment by means of an articular membrane, a region in which the cuticle is very thin and flexible. Basically, the cuticle of each segment is divided into four primary plates—a dorsal tergum, two lat¬eral pleura, and a ventral sternum (Fig. 1). This pattern has frequently disappeared because of either secondary fusion or subdivision.
The cuticular skeleton of the appendages, like that of the body, has been divided into tubelike segments, or sections, connected to one another by articular membranes, thus creating a joint at each junction. Such joints enable the segments of the appendages, as well as those of the body, to move (hence the name of the phylum, Arthropoda—jointed feet). In most arthropods the articular membrane between body segments is folded be¬neath the anterior segment. In| arthropods the additional development condyles and sockets is suggestive of ventral skeletal structures.