228 



ANNELIDA AND ARTHROPODA: 



the plankton feeders which filter microscopic organ- 

 isms (plankton) from water. This group is dominated 

 hy xhc'^ feather-duster worms'' which have feathery gills. 

 Then, there is Aphrodite, the sea mouse. This is an oval 

 animal about 3 inches long whose sides are covered 

 by irridescent, flexible bristles. The colorful bristles 

 of this filter feeder are often not noticed, owing to the 

 debris that covers the animal. Even with its colorful 

 bristles, however, the animal is an insult to the god- 

 dess of love. Next, are the many kinds of delntus 

 feeders. The tentacle worms use their mucous-covered 

 tentacles to collect living and nonliving material from 

 the surface of the mud. Most of these worms make 

 leathery tubes in permanent burrows in mud flats or 

 on the ocean bottom. A second method of detritus 

 feeding is found in the lugworm which vaguely re- 

 sembles an earthworm with a bulb on the front end. 

 There is some selection of food materials by the 

 mucous-covered bulb as the animal burrows through 

 sand or mud. A third method of detritus feeding is 

 most characteristic of certain fan worms, so-named 

 because of the enlargement of certain side appendages 

 at about midbody. These fans are used to create a 

 current through a mucous bag that is formed by the 

 worms. Fan worms live in U-shaped burrows in sand 

 or mud and somewhat resemble certain echiuroids in 

 this respect. Other fan worms have single-opening 

 burrows and rely on mucus on the body rather than 

 forming a bag. There is also a group of detritus- 

 feeding feather-duster worms. The third main type of 

 feeders are the predaceous worms, a group including 

 more than one kind of worm. Among these, the 

 nereids, or dam-worms, are somewhat snake-like in gen- 

 eral appearance. These worms seize their prey with 

 the two hook-like teeth at the end of a short, eversible 

 proboscis. The glycertds, or proboscis worms, resemble 

 the nereids but have four hook-like teeth at the end of 

 a long eversible snout (proboscis). The polynoids, or 

 scale worms, have the entire back covered by scales and 

 have teeth at the end of a long eversible proboscis. 

 The fourth, and last, feeding type is represented by 

 the seaweed feeders. This feeding type is represented 

 by certain, large burrowing nereids. 



In most polychaetes the sexes are separate. Fertili- 

 zation is accomplished by spawning of eggs and 

 sperm into the water. Spawning is in unison and, in 

 many species, is dependent upon lunar cycles, tides, 

 temperature, season of the year, time of day, light, 

 and calmness of the water. Asexual reproduction in 

 the form of budding is known. 



This is another group that has long been used by 

 primitive man as a source of food. Many peoples of 

 the Pacific Islands gather worms, especially during 

 the spawning congregations of some worm species. 

 Worms taken during the reproductive season are 

 considered especially tasty. Polychaetes are used as 

 food by many marine animals, a fact which causes 

 many of these worms to be used as fish bait by man. 



CLASS OLIGOCHAETA (Earthworm and allies) 



Diagnosis: bristles (setae) involved in locomotion, 

 but relatively few in number; no parapodia and head 

 much reduced; collar (clitellum) present; hermaphro- 

 ditic, eggs often in cocoons; without larval stages 

 (Figure 14.2). 



Figure 14.2 Class Oligochaeta; above, lumbricus, on earthworm, 

 subterranean and terrestrial; below, left, Tubifex, o bristleworm, fresh- 

 water with reddish tail vibrating from mud tube; below, right, Chae/o- 

 goster, a bristleworm, fresh water. 



In contrast to the polychaetes, the earthworms and 

 their relatives are primarily land and fresh-water 

 animals. The free-living oligochaetes obtain food by 

 taking in large amounts of the substrate in which they 

 live. The food items are digested and absorbed from 

 this material and the rest passes out of the digestive 

 tract. The branchiobdellids, or commensal oligo- 

 chaetes, live upon the external surface of crayfishes, 

 usually on or around the gills. They are thought to 

 be commensal, rather than parasitic, because their 

 digestive tracts are known to have diatoms, certain 

 protistans, and organic debris. 



Oligochaetes are mostly hermaphroditic, but cross- 

 fertilization is the usual case. Asexual reproduction 



