THE ANIMAL KINGDOM 9 



have no true circulatory system, but the pseudocoelom distributes digested 

 food and collects wastes. A few possess a closed vascular system. Excre- 

 tion is effected by protonephridia devoid of flame-cells. The sexes are 

 usually separate. 



Nemathelminths are subdivided into three classes, a. Nematodes or 

 round worms; b. Acanthocephala or hook-headed worms; c. Chaetognatha 

 or arrow-worms. 



Sixteen hundred species are recognized, mostly parasitic. The thick- 

 ened cuticula on the outside of the body is probably an adaptation to 

 their parasitic mode of life. 



Phylum 6. MOLLUSCOIDA 



Molluscoids are animals of ancient ancestry, and the bivalve "lamp- 

 shells" or brachiopods are abundant in the older sedimentary rocks. 

 Living forms are mostly sessile and marine. The valves of the shell, 

 instead of being lateral and paired, as in the clam, are dorso-ventral. 

 The ventral valve is the more curved of the two, and encloses posteriorly 

 an opening for the stalk by which the animal is attached. 



Molluscoids are coelomate, and the body cavities are bilaterally paired. 

 In them are enclosed the alimentary canal, the liver, and the gonads. 

 Transverse septa divide the coelom into three portions, as in Hemi- 

 chordates and in Sagitta. While some species lack an anus, most have 

 a U-shaped alimentary canal with the anus near the mouth. Paired 

 nephridia function both as excretory and as reproductive outlets. The 

 nervous system consists of a circumesophageal ring with dorsal and ventral 

 nerve chains. The heart is dorsal to the stomach. Some forms, such 

 as the bryozoa, are colonial. 



The phylum is subdivided into three classes: Polyzoa, Brachiopoda, 

 and Phoronida. 



The origin of molluscoids from free-swimming trochophore ancestors 

 is generally assumed. 1700 species have been identified. 



Phylum 7. ROTIFERA 



The rotifers are so named because the cilia that surround or encircle 

 the mouth appear under the microscope to rotate like a wheel. They are 

 little known, partly because of their small size, and partly because they 

 are often confused with ciliated protozoa which they superficially resemble. 

 They are, however, multicellular unsegmented worms with a pseudo- 

 coelom. The alimentary canal has both mouth and anus. Usually, 

 the protonephridia and gonadic ducts open into the posterior portion 

 of the intestine. The sexes are separate. Rotifers are frequently com- 



