Zool.-Vol. I.] JOHNSON— PACIFIC COAST ANNELIDS. 171 



nutely papillated, pigmented nearly their entire length, darkest adjacent to 

 the white tip. 



Nephridial papillae begin usually at eighth somite; sometimes as far back 

 as the twelfth; end at thirty-fifth or thirty-sixth. 



Colors never strong or bright, varying from reddish brown to ashy, gray, 

 or drab, sometimes very pale. A small, darker spot on each elytron, fre- 

 quently accompanied and accentuated by a light spot, as in P. brevisetosa. 

 A double row of rectangular dark spots, often present on ventral side, seg- 

 mentally arranged. Each dorsal cirrus, also tentacle, antenna?, peristomial 

 cirri, and anal cirri, ringed with a dark band at or about the level of the 

 thickened area. Dorsum transversely barred with alternate dark and light 

 bands. 



Measurements.— Large specimen: length, 48 mm.; greatest breadth, 

 7.25 mm. 



The present species is distinctly southern in its range, 

 occurring abundantly at San Pedro and at San Diego. I 

 have found a few individuals at Pacific Grove, but invaria- 

 bly in the tubes of Ainfthitrite or Tkeleptis — never free-living. 

 I have not found it further north. 



The commensal habit is no more established in this species 

 than in P. brevisetosa; both are facultative commensals, as 

 far as the species is concerned, but probably not entirely so 

 as regards individuals. The constancy of the commensa- 

 listic life throughout an individual existence is evinced both 

 by the invariable commensalism of P. reticulata at the 

 northern limit of its distribution, where it comes into compe- 

 tition with P. brevisetosa, and by the thorough-going modi- 

 fications caused by this mode of life in the latter species. 



CEcologically, P. reticulata almost exactly replaces P. 

 brevisetosa beyond the southern limit of the latter, and the 

 superficial resemblance of the two species is very strong. 

 It fills the same place in nature, crawling over stones, 

 nestling under them, or in the countless crevices among the 

 multifarious vegetable and animal growths of the " littoral 

 belt." Further than this, not a few individuals are found 

 in the tubes — whether occupied by the rightful owner or 

 not — of Terebellid worms, mainly species of Amfhitrite, 

 Terebella, and Thelepus. The commensal habit has by no 

 means brought about such modifications in the present 

 species as in P. brevisetosa. I have not found it easy to 



