210 INVERTEBRATE MORPHOLOGY. 



individuals; but in the majority of species they do not seeni 

 to be developed. 



Ciliated depressions which have been supposed to be 

 olfactory have been described as occurring in the anterior 

 region of the body in various species, reaching a high devel- 

 opment in the Capitellidse, where they form club-shaped sacks 

 capable of being evaginated. In addition there are to be 

 found scattered on the surface of the body minute beaker-shaped 

 depressions, at the bottom of which are cells bearing long 

 hairs and presumably sensory in function ; and furthermore 

 in a few forms, such as the Capitellidae and Polyophthalmus, a 

 series of sensory hillocks occur along the sides of the body 

 a pair in each metamere, forming the sense-organs of the lateral 

 line. In the Capitellidse these organs are in the anterior inet- 

 ameres contained in depressions, but more posteriorly they 

 project slightly from the surface. The central part of each 

 projection is retractile and is formed of a number of hair-cells, 

 each of which is in connection at its inner end with a nerve- 

 fibril. No little interest attaches to these organs, which 

 forcibly recall, both in their structure and distribution, the 

 lateral line organs of the lower Vertebrates. 



The nephridia (Fig. 99, ne), in typical adult forms, occur 

 as a single pair in each metamere except the two terminal 

 ones. Each consists of a usually contorted or coiled tube 

 lined with cells opening by a funnel-shaped mouth into the 

 ccelom of the metamere, perforating the dissepiment between 

 it and the next metamere in which the greater portion of it 

 lies and in which it opens to the exterior by a small pore sit- 

 uated on the ventral surface of the body at the base of the 

 parapodium. It is rare, however, that any such metarneric 

 regularity of arrangement occurs, and very frequently they 

 become reduced to a small number, or even to two pairs ; in 

 the tubicolous forms a few pairs are frequently found in the 

 anterior portion of the body much larger than any of the rest. 

 In addition to their original excretory function they may also 

 serve as outlets for the reproductive elements, and in some 

 cases become specially modified for this purpose and lose 

 their original function. 



