The annelid excretory system consists of a 

 series of well-developed nephridia (Fig. 20- 

 3). Typically there is a pair of nephridia per 

 segment, each supplied by a network of 

 blood capillaries (p. 371). Respiration takes 

 place through the skin (p. 358), although 

 some tube-dwelling Annelida have gills. 



The well-developed nervous system of the 

 annelid is very similar to that of the arthro- 

 pod. Essentially, the annelid system (Fig. 25- 

 5) consists of a double chain of segmental 

 ganglia, which extends longitudinally along 

 the midline, on the inner surface of theventral 

 body loall. The chain connects anteriorly 

 with the two dorsally placed cerebral ganglia 

 by way of a pair of nerve connectives, the 

 circumpharyngeal connectives, which encircle 

 the digestive tract (Fig. 25-5). In each seg- 

 ment, there are several bilaterally matched 

 nerves that transmit impulses from the re- 

 ceptors and to the effectors of this particular 

 part of the body. 



Classes of the Phylum Annelida. The fol- 

 lowing four classes were evolved among the 

 Annelida: 



Class 1. The Archiannelida (Literally, "An- 

 cient Annelids"). These are all small marine 

 annelids, usually without any parapodia or 

 setae; segmentation is inconspicuous exter- 

 nally, but well-defined internally. 



Class 2. The Polychaeta (Literally, "Many 

 Bristles"). These are the sandworms, tube- 

 worms, etc.; predominantly a marine group 

 with numerous conspicuous segments, each 

 bearing a pair of many bristled paddle-like 

 parapodia, which serve as swimming organs. 

 A well-defined head, with tentacles, is char- 

 acteristic (Fig. 32-26). 



Class 3. The Oligochaeta (Literally, "Few 

 Bristles"). This group includes the earth- 

 worms and related forms. They live mainly 

 in fresh water or moist soils; have conspicu- 

 ous segmentation, no obvious head, and only 

 a few setae per segment. 



Class 4. The Hirudinea, or Leeches (Fig. 32- 

 27). These annelids have a dorsoventrally 

 flattened, pigmented body, with a large 

 sucker at the posterior end, and (usually) a 



The Animal Kingdom - 649 



smaller anterior sucker, surrounding the 

 mouth. Externally the body is marked by 

 many transverse creases, but a study of the 

 internal structure reveals only 34 segments. 

 No parapodia are present; a distinct head is 

 lacking; and only one species possesses setae. 

 Most species live in quiet fresh or salt waters; 

 but a few live in damp soil. 



Relationship of the Annelida to Other 

 Phyla. Annelida shows some resemblances to 

 other phyla, especially the Mollusca, Arthro- 

 poda, and Chordata. The pattern of cleav- 

 age, the manner in which the mesoderm 

 arises, and certain larval characteristics are 

 reminiscent of the Mollusca. But the seg- 

 mentation, the type of cuticle, and the struc- 

 ture of the nervous system all tend to follow 

 the arthropod pattern; and the conspicuous 

 coelom and definite segmentation resemble 

 these features in Chordata. However, the 

 early paleontological record is not clear as to 

 the precise origins of any of these important 

 groups. Most students think that all orig- 

 inated from a common stock subsequent to 

 the development of a coelomic cavity; that 

 the Mollusca branched off before segmenta- 

 tion developed; and that the Annelida and 

 Chordata began to diverge shortly after seg- 

 mentation appeared in the ancestral animal 

 stock (Fig. 29-11). 



THE MOLLUSCA 



Mollusks constitute the second largest phy- 

 lum in the animal kingdom. The group, in- 

 deed, comprises more than 40,000 living 

 species; and fossil remnants of more than 

 35,000 extinct species have been identified 

 at the present time. The many diverse species 

 include: all clams, oysters, and scallops; 

 the chitons, slugs, and snails; and the squids, 

 octopuses, and nautiluses. Also the size range 

 is very great. Some snails measure only 1 mm 

 in diameter; but one great giant squid 

 reaches a length of 60 feet and a weight of 

 several tons — easily the record among in- 

 vertebrate animals. 



Phylum Characteristics. As is shown in 



