SUBPHYLUM MYRIAPODA The term “myriapod,” meaning “many footed,” describes members of four classes in subphylum Myriapoda that have evolved a pattern of two tagmata—head and trunk—with paired append ages on most or all trunk segments. Myriapods include Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda (pauropods), and Symphyla (symphylans). Myriapods use tracheae to …

Order Opiliones: Harvestmen Harvestmen, often known as “daddy longlegs,” are common throughout the world and comprise about 5000 species. They are easily distinguished from spiders: their abdomen and cephalothorax are rounded and broadly joined, without the constriction of a pedicel; their abdomen shows external segmentation; and they have only two …

Order Scorpiones: Scorpions Scorpions are perhaps the most ancient of terrestrial arthropods and comprise about 1400 species worldwide. Although scorpions are more common in tropical and subtropical regions, some occur in temperate zones. Scorpions are generally secretive, hiding in burrows or under objects by day and feeding at night. They …

Arachnids (Gr. arachne¯, spider) exhibit enormous anatomical variation. In addition to spiders, the group includes scorpions, pseudoscorpions, whip scorpions, ticks, mites, daddy longlegs (harvestmen), and others. There are many differences among these taxa with respect to form and appendages. They are mostly free-living and are most common in warm, dry …

Class Pycnogonida: Sea Spiders About 1000 species of sea spiders occupy marine habitats ranging from shallow, coastal waters to deep-ocean basins. Some sea spiders are only a few millimeters long, but others are much larger with legspans up to nearly 0.75 m. They have small, thin bodies and usually four …

Class Hirudinida is divided into three orders, Hirudinea, the “true” leeches, and two others that merit mention here because their members are morphological intermediates between oligochaetes and true leeches. Oligochaetes have variable numbers of segments, segments bear setae, and there are no suckers on the body. True leeches have 34 …

Phylum Sipuncula (sigh-pun + kyu-la) (L. sipunculus, little siphon) consists of about 250 species of benthic marine worms, at depths ranging from the intertidal to over 5000 m. They live sedentary lives in burrows in mud or sand, occupy borrowed snail shells, or live in coral crevices or among vegetation. …

Phylum Echiura (ek-ee-yur a) (Gr. echis, viper, serpent, oura tail, ida, pl. suffix) consists of about 140 species of marine worms that burrow into mud or sand, live in empty snail shells or sand-dollar tests, or rocky crevices. They are found in all oceans—most commonly in littoral zones of warm waters—but …

The first molluscs probably arose during Precambrian times because fossils attributed to Mollusca have been found in geological strata as old as the early Cambrian period. On the basis of such shared features as spiral cleavage, mesoderm from the 4d blastomere, and trochophore larva, many zoologists argue that Mollusca are …

Class Oligochaeta More than 3000 species of oligochaetes are found in a great variety of sizes and habitats. They include the familiar earthworms and many species that live in freshwater. Most are terrestrial or freshwater forms, but some are parasitic, and a few live in marine or brackish water. With …