270 



PRINCIPLES OF ANIMAL BIOLOGY 



the palolo of Samoa and other Pacific islands. These worms burrow 

 in the coral reefs, and swarm in the open water in huge numbers just 

 before the last quarter of the moon in October and November. They are 

 captured in quantity by the natives at that time. 



Fig. 226. — Autolytus, a 

 marine worm. 



Fig. 227. — Aeolosoma. 

 a fresh-water worm. 



Fig. 228.— a leech. (From 



Leunis.) 



Some of the annelida live in the soil (earthworms), many live in fresh 

 water (Figs. 227, 228), and many are marine. Some of the leeches (Fig. 

 228) attach themselves to vertebrate animals by means of suckers and 

 feed upon the blood of their host. Chaetopterus (Fig. 229) lives in a 



Fig. 229. — Chaetopterus. {Courtesy of Carolina Biological Supply Co.) 



U-shaped parchmentlike tube in the sand under marine waters. The 

 tube is open at both ends, and circulation of the water in it is maintained 

 by flat appendages at the sides of the body. 



Class I. Archiannelida. Marine Annelida with no setae or parapodia. 

 Class II. Chaetopoda. Annelida with setae and a perivisceral coelom; marine, 

 fresh-water, or terrestrial in habitat. Earthworms. 



Subclass I. Polychaeta. With many setae. Marine worms. (Figs. 226, 



229.) 



Order 1. Phanerocephala 



Subclass II. Oligochaeta. 

 worms. (Figs. 135, 136, 227.) 



Order 1. Microdrili 



Order 2. 

 With few setae. 



Cryptocephala 



Fresh-water and terrestrial 



Order 2. Macrodrili 



Class III. Hirudinea. Annelida without setae, and with anterior and posterior 



suckers. Leeches. (Fig. 228.) 



Phylum 8. MoUusca. — This group includes clams, snails, and cuttle- 

 fishes. Their structure is so diverse that the phylum is difficult to 

 define. Mollusks are triploblastic, unsegmented, and bilaterally sym- 

 metrical, though their symmetry is disturbed by a secondary spiral 

 winding in some kinds. They have a coelom of restricted extent and 

 usually possess a shell. The name moUusk refers to their soft bodies. 



A structure called the foot is characteristic of the phylum but varies 

 greatly in form. In the chitons (Fig. 230) it is like the sole of a shoe. In 



