Chap. 27 



AN AQUATIC MISCELLANY 



537 



Fig. 27.4. Reproduction by natural division in Linens socialis. A, mature worm; 

 B, dividing; C, reconstruction of these pieces into nine complete worms; D, por- 

 tion of body of mature worm showing zones of division. (Courtesy, Coe, 

 Physiol. ZooL, 3:299, 1930.) 



Structures and Functions. The important structural advances in which rib- 

 bon worms have progressed beyond the flatworms are the digestive canal with 

 mouth and anal openings present in all members of the phylum, and the circu- 

 latory system. The general plan of the body is otherwise similar to that of 

 planarians. The body is completely covered with ciliated epithelium and be- 

 neath it are the circular and longitudinal muscles. There is no special respira- 

 tory organ. The excretory system consists of a pair of lateral canals with side 

 branches ending in flame cells. The male and female systems are usually in 

 separate individuals; a few species are hermaphroditic. Eggs and sperm are 

 produced in many little sacs which open directly to the outside. The sex cells 

 are strewn into the water where fertilization occurs and the free-swimming 

 helmet-shaped larva (pilidium) develops (Fig. 27.1). 



Ecology. Habitats. Most marine ribbon worms are bottom dwellers in 

 mucky sand, within holes lined with mucus; some live in parchmentlike tubes 

 similar to those built by annelid worms. 



Feeding. Ribbon worms are carnivores — burrowers that feed chiefly on 

 annelid worms, especially the abundant clamworms (Nereis), and they forage 

 mostly at night when the latter are active. The proboscis is their chief burrow- 

 ing tool. 



Way of Living. Many nemerteans are free-living predators. Others are 

 commensal, sharing "the bed and board" with a partner without affecting it. 

 A smaller species {Malacobdella grossa) lives in the mantle cavity of various 

 clams on the Atlantic coast, and in about 80 per cent of the razor-shell clams 



