ANIMALS WITH JOINTED FEET-THE ARTHROPODS 



203 



ing as they are rare, being found only 

 occasionally in widely separated places — in 

 Africa, Australia, Asia, and the two Ameri- 

 cas. Because they are limited to widely 

 scattered, isolated places, they are thought 

 to have come from the annelid-arthropod 

 stock at a very early period in biological 

 history, subsequently dying out in the areas 

 between. Peripatus lives in the dense, tropi- 

 cal forests, in damp places under logs and 

 other objects found on the forest floor. The 

 inexperienced observer would mistake it for 

 a caterpillar because of its long, many- 

 appendaged soft body with anteriorly pro- 

 truding antennae (Fig. 11-2). Like anne- 

 lids, Peripatus has a pair of appendages 

 for nearly every segment. These have 

 clawed feet but they lack joints, hence un- 

 like the typical arthropod appendage. The 

 exoskeleton is also very thin, like the cuticle 

 of annelids and very unlike the usual ar- 

 thropod. 



The excretory system is definitely anne- 

 lid-like, while the nervous system is even 

 more primitive than that found among the 

 annelids. The occurrence of cilia in the 

 reproductive tubules is also an annelid 

 character, since such structures occur 

 nowhere else among the arthropods. On 

 the other hand, its circulatory system and 

 coelom are similar to those found in other 

 arthropods. The respiratory system re- 

 sembles that of the arthropods but is suf- 

 ficiently different to be thought of as having 

 arisen independently. 



It is apparent that Peripatus is, in many 

 ways, intermediate between annelids and 

 arthropods. This evidence seems to point 

 unmistakably to the annelids as the arthro- 

 pod ancestor. Of course, Peripatus is not 

 identical with the early arthropod, since it 

 must have gone through changes itself dur- 

 ing this very long period of time. However, 

 it seems to have changed but little by com- 

 parison to other present-day members of 

 the phylum, and it seems safe to say that it 

 probably resembles the early arthropod 

 more than any other living form. 



(Photo by Ralph Buchsbaum) 



Fig. 11-2. Peripafus is a rare animal that shows both 

 annelid and arthropod characteristics and for that 

 reason is of considerable interest to zoologists. 



Peripatus is thus a most remarkable ani- 

 mal. Some zoologists now place it in a 

 separate phylum. It is a pity that it cannot 

 be studied in the beginning zoology labora- 

 tory, but it is scarce, hence costly. 



THE CRUSTACEA 



The Crustacea make up a large and suc- 

 cessful class of arthropods. They live on 

 land, in fresh water (Fig. 11-3) and in 

 the ocean (Fig. 11-4). They include such 

 common forms as fresh-water fairy shrimps 

 and crayfish (Fig. 11-5), seashore crabs 

 (Fig. 11-6), lobsters, and barnacles (Fig 

 11-7). The group is of considerable eco- 

 nomic significance to man. It is a valuable 

 source of food, as demonstrated by the fact 

 that around $10 million worth of shrimp, 

 crab, and lobster are marketed annually in 

 this country alone. The large leg muscles 

 and the choice abdominal muscles of the 



