SEGMENTED SCHIZOCOEIS 



229 



by budding occurs in a few oligochaetes, as it does in 

 a few polychaetes. 



Primitive peoples fiave used and still use oligo- 

 chaetes as food. They also are widely used as fish 

 bait. Perhaps, their greatest benefit to man is the 

 making of burrows in the soil. This earthworm ac- 

 tivity allows greater aeration of the soil and also 

 contributes to soil formation because the earthworms 

 cast wastes upon the ground surface. 



CLASS HIRUDINEA (Leeches) 



Diagnosis: without bristles (setae), parapodia, or 

 collar (clitellum), and head much reduced; anterior 

 and posterior suckers present and used for attach- 

 ment; hermaphroditic, producing eggs in cocoons but 

 no larval stage; many are external, blood-sucking 

 parasites (Figure 14.3). 



Figure 14.3 Class Hirudinea, Macrobdello, the common bloodsucker 

 or American leech, fresh-water. 



Most leeches are found in fresh water, but a few 

 live upon marine fishes and some in moist places on 

 land. Fresh-water species are most numerous in 

 warm, protected shallows where there is little water 

 movement and plants, rocks, and debris offer conceal- 

 ment. Because they require attachment surfaces, they 

 are seldom found on mud or clay bottoms. Leeches 

 are either scavengers, predators, or parasites. The 

 scavengers rely mostly on animal remains; the preda- 

 tors most commonly eat small worms, insects, and 

 moUusks; and the parasites are most common on 

 crustaceans and vertebrates. 



Leeches are hermaphroditic, but cross-fertilization 

 is the practice. Most eggs are protected by a cocoon 

 that is placed in water or in earth. In some species, 

 the eggs are laid on rocks. Some carry their develop- 

 ing eggs on the underside of their bodies. 



Leeches are food for various aquatic animals, 

 especially fishes; hence, they are another source of 

 fish bait. In our part of the world they are little more 

 than a nuisance to swimmers, but some of the land 

 leeches of Asia and the East Indies can cause severe 

 injury to man. Most famous of the leeches is Hirudo 



medicinalis, the medicinal leech. This animal once was 

 used in the practice of blood-letting. Today these 

 leeches are sometimes used to treat "black eyes." 



SEGMENTED, JOINT-LEGGED 

 SCHIZOCOELA: ARTHROPODA 



The primary difference between segmented worms 

 and joint-legged animals is indicated by the common 

 name of the arthropods. Annelids have no jointed 

 legs and arthropods do. However, their similarity is 

 shown by a living "missing link," Peripatis (Sub- 

 phylum Onychophora), the joint-legged worm. Now, 

 you should realize that the idea of a "missing link" 

 breeds a great deal of misunderstanding. How might 

 one describe the annelid-arthropod characters in 

 Penpatus, when Penpalus probably originated as a 

 species much later than the time of annelid or arthro- 

 pod origin? 



ARTHROPODA (Joint-legged Animals) 



Diagnosis: symmetry bilateral; segmentation visi- 

 ble externally in varying degrees; appendages are 

 jointed; schizocoelous; possess a hard, external skele- 

 ton, containing a chemical compound called chitin; 

 probably more numerous than all other life com- 

 bined; marine, fresh-water, terrestrial, aerial, and 

 parasitic. 



To discuss the arthropods in a few pages is akin to 

 writing an encyclopedia on the head of a pin. The 

 joint-legged animals are the largest and most diverse 

 of all animal phyla. In fact, one single class, the 

 Insecta, includes more than half of the known species 

 in the animal kingdom. If any animal group today 

 rules the earth, the arthropods must be acclaimed as 

 masters. Let us then inspect these terrible conquerors 

 to discover whence they came and what they are. 



We may consider arthropods as basically annelid- 

 like animals grown up in their evolutionary potential- 

 ities. Recall some of the features of annelid structure: 

 segmentation, or metamerism, in which most seg- 

 ments are alike; a well-developed, segmental, ladder- 

 type nervous system; simple, bristle-like locomotor 

 appendages, the setae. Add to these the principle of 

 cephalization, and evolution is well on the way to the 

 arthropods. Cephalization is the evolutionary ten- 

 dency in freely moving animals toward concentration 

 of feeding structures, special senses, and nervous 



