333 



Sphaeroma pentodox Richardson 



This is an organism averaging about 1 cm. in length, and about half as broad as 

 long. Its color is dark olive to slightly reddish brown. It is often mottled or l)lotched 

 with lighter dull yellowish areas on the middle of the back. The eyes are prominent 

 and lateral in position. The surface of the body is minutely but densely granular. 

 The posterior segment is evenly rounded in outline, and bears on its median surface 

 two sub-parallel rows of four tubercles each. There is a prominent transverse elevation 

 across its posterior border. 



The outer mobile branch of the last pair of abdominal appendages bears five 

 posteriorly directed denticulations, which serve the useful purpose ot taking hold of 

 the sides of the burrow and pushing or holding the head and gnawing mouth parts 

 firmly against the bottom of the burrow. 



The only organism with which this is likely to be confused is Exospluicronia 

 oreg,07iensis, a related form whith occurs commonly on the surface of piling in .San 

 Francisco Bay, and even in the burrows of Sphaeroma, but is not known to do any 

 boring. This organism is similar in shape and appearance to Sphaeroma pentodon, 

 but it lacks the regularly arranged excrescences on the terminal segment ol the ab- 

 domen, and it does not have teeth on the outer ramus of the uropod. 



DisTRiiu'Tiox ,\XD Habits 



Sphaeroma pentodon (fig. 1,^7) is generally ilistributed throughout .San Francisco 

 Bay, and may be anticipated at any locality, in salinities ranging from that of normal 

 sea water down to that of waters that are only faintly Ijrackish. It has been lound 



Fig. 137. Dorsal, lateral and \x-iitral \ie\vs of Sphaeroma pentodon. Richartison. 



to occur at practicalh- e\ery point in the ba>- where search for it has been made, 

 although damage by it at most localities is negligible. It occurs most numerousK' 

 in the upper portions of San Pal)lo Bay, and its activities are extended some distance 

 up from the mouths of the small streams that empty into the bay here. Thus it occurs 

 well up in Xapa Creek, and up Petaluma Creek at least as far as the Northwestern 

 Pacific drawbridge just below the city of Petaluma. Its acti^■ities in test boards placed 

 at this bridge are shown in figure 137. The organism also occurs throughout Suisun 

 Bay, and as far upstream as Antioch, and probably beyond. 



A frequent, and probably the normal, habitat of this species is clay and friable 

 rock (Barrows, 1919). Its burrows occur in great numbers in a peat formation along 



