THE INVERTEBRATE LEGIONS 111 



travel in soft or loose bottom. The lugworm, Arenicola, is common in 

 temperate seas and reaches about a half-foot or more in length. 

 3. Tube-dwelling Polychaetes. The beautiful fan worms of the family 

 Sabellidae, plankton-feeding worms with a delicate crown of plumes at 

 the head end CcoJor photooraph^, belong to this group. They are found 

 in all seas but are largest in the tropics, where their flowerlike tentacles 

 are often seen protruding from tubes in massive corals. When disturbed, 

 they quicklv retract into their tubes. 



The tubes, which are secreted by these worms, may be hard and calcified, 

 as they usuallv are when in exposed places, but some of these worms 

 live in U-shaped, parchmentlike tubes in soft bottoms. One of these is the 

 parchment worm, Chaetopterns, of temperate seas. It is among the most 

 luminescent of animals and is extremelv delicate. It circulates water through 

 its tube bv flapping delicate appendages in order to filter-feed on plankton. 



Sipunculids: Class Sipunciilida — Figure 35 



These are large, sand- or mud-dwelling, burrowing worms of most seas. There 

 is no segmentation to the muscular body. Thev live- in burrows and have at 

 the head end a retractile, tentacle-bearing, muscular "introvert" with which 

 they grasp and swallow mud and sand. Thev are, therefore, much like earth- 

 worms in habits. 



Echiurids: Class Echiurida — Figure 35 



These worms are very much like sipunculids, but instead of an introvert, they 

 have a trunklike extension for feeding. Echmrius is a stout-bodied species which 

 lives in a U-shaped tube in mud. 



The larvae of the European echiurid, BoiielUa, have an unusual method of 

 establishing which sex they will be. The larvae are free-swimming. If they land 

 on the trunk of an adult female, they develop into small, parasitic males on 

 that female. If they do not land on a female, they grow into solitary females. 



The trunk of echiurids serves both for locomotion and for food-getting. 

 Some species live in soft bottoms and some in clefts in rock, and they frequently 

 move from place to place, not staying in one cleft or tube. They do not eat 

 sand or mud, as do sipunculids, but eat animal and plant matter, probably 

 mostly debris. 



ARTHROPODS: Phylum Arthropoda ("joint-legged") 



Four-fifths of all species of animals on earth are arthropods (insects, crusta- 

 ceans, spiders, etc.). They literally abound everywhere, in every imaginable 

 habitat, occupying more different kinds of habitats than any other phylum. For 

 these reasons, arthropods are undoubtedly the most successful animals on earth. 



Arthropods are named for their many-jointed appendages. In the primitive 

 species, there is a pair of appendages present on every segment of the body, 

 but in the advanced species, these appendages look very different in different 

 parts of the body, being modified into antennas, various mouth parts such as 



