324 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 10 



to solid objects, such as shell and stones. They are largely intertidal or 

 littoral, subject to a varying amount of exposure through tidal action, but 

 a few species have been recorded from abyssal depths (some species of 

 Lygda?nis). There is direct proportion between the degree of exposure 

 endured and the extent of opercular development. In species of Phragtna- 

 topoma (p. 346) often existing high up near the barnacle zone (pi. 42) 

 the operculum is a firm, conical structure capable of sealing the open end 

 of the tube, specially adapted to prevent desiccation. In other species the 

 opercular spines are progressively less adapted for this purpose but per- 

 haps increasingly so for protection against predatory organisms. Geo- 

 graphically, the Sabellariidae are widely distributed, but the majority of 

 species abound in warmer seas. 



The body of the Sabellariidae consists of ( 1 ) an anterior end some- 

 times called the opercular disk and stalk, (2) a thorax, (3) an abdomen, 

 and (4) a caudal tail (=cauda). Differentiation is sharp in each of these 

 regions. The total number of larval somites which come to form the 

 adult body is variable and perhaps great, but difficult to determine because 

 of the high degree of specialization. Even a study of larval stages is inade- 

 quate to satisfactorily settle these problems, since metamorphosis is veiy 

 pronounced and comparatively rapid. Several important studies have been 

 made toward the elucidation of problems concerning the origin and 

 homology of specialized parts. The investigations of Quatrefages ( 1848), 

 Meyer (1887, 1888), and Johansson (1927) have been foremost in 

 clarifying some of the issues. The important studies by Wilson (1929) 

 on the development and metamorphosis in some species should also be 

 stressed, but are based largely on external manifestations during larval 

 and settling stages. They include also interesting observations on the 

 structure of the caudal appendage and the organs involved in the process 

 of tube building. 



The origin of the conspicuous opercular stalk, constituting the crown, 

 has been explained in several ways. Fauvel (1927, p. 205) considers the 

 operculum probably "les rames dorsales fusionnees des deux premiers 

 pieds." Perhaps this means only the 2 parapodia of the first pair, and not 

 the first 2 pairs. The 2 fascicles of capillary setae, near the oral aperture, 

 and those on the following segment are considered to correspond to the 

 ventral rami. These conclusions correspond somewhat with those proposed 

 by Meyer (1887, 1888). Johansson's conclusions (1927, pp. 25-39), based 

 on a careful study of the nervous system and other anatomical parts, are 

 of great interest and significance; they differ from those of Meyer in 

 several important respects. Meyer concluded that the opercular paleae 



( 



