move out, as I believed, bead first; but a nearer inspection showed me that 

 the motion was sideways, and t' at the new-comer was no fourteen-legged 

 amphipod or isopod, but a true decapod crustacean. 



The short chehe, extremely lengthened fourth pair of legs, aud short, broad 

 body, are so many adaptations to the mode of life of this creature, which 

 finds an ample dwelling-place in the space intervening between the body of 

 the annelid and the inside of the tube; up and down which it moves with its 

 long fourth jaair stretched oat in such a manner as to to give it the elongated 

 appearance of a CaprelUi. 



The width of Tubieoln long! pes from end to end if the fourth pair of legs is 

 eight times greater than its length from front to back. 



This is, so far as I am aware, the only instance known of a decapod crus- 

 tacean becoming the guest or commensal of an annelid, for althoiigh the 

 species of the family Pinnotheridf are all commensals, most of them reside 

 between the folds of the mantle of large bivalve mollusks, such as mussels 

 or clams (thus the Fabia suhquadrata lives within the mantle of Pachydesma 

 crassite/loides. a large clam of this Coast), and a few live within the tests of 

 Echini, close to the anal aperature. 



It is most probable that this worm and its commensal may occur in many 

 other places besides Tomales Bay, possibly in San Francisco Bay, and I should 

 be much obliged if some of our friends who may go out on a fishing excursion 

 would bring me specimens, in alcohol, of the worm and its tubes, that I may 

 find whether the crab is its constant companion in all localities. 



The worm is one which is frequently used for bait. 



On April 2Uth, the females of this interesting little crustacean was loaded 

 with spawn. 



Phyllodurus abdomhia'is. Siimpson. 



When Stimpson, in his Crustacean aud Echinodermata of the Pacific Shore 

 of N. A., page 71, first de-eribed this species, the female only was known to 

 him. This female, like all those belonging to the family Bopyridce is of com- 

 paratively large size, broad and ilumsy in appearance, and lives attached to 

 another crustacean. 



The crustacean frequented by this commensal is Geb'n piiyettensis, a marine 

 crayfish common on these shores. 



About April '24th, I gathered a great number of Gebias in Tomales Bay, and 

 found that most of them, all except the largest speciaiens, had a female P- 

 abdonvnalis attached to one of the abdominal pairs of feet, to which it clung 

 closely by means of its hooked claws. 



A close inspection revealed, beside or near the lai'ge female, a small and 

 slender male, a kind of miniature edition of its stout mate. 



Never more than a single pair were ever found attached to one Gebia, but 

 the males appeared so regularly to accompany the females, that I believe that 

 in the few cases I did not find them, it was because they had dropped off in 

 handling the specimens. 



The males do not live attached to the Gebia, but are free to rove, and their 

 constant presence at this season by the side of the females proves that this 

 is their season of love. 



