434 Chas. W. Hargitt 



In 1873 G. 0. Sars reported a new species ^ wliicli be took from 

 a clepth of 400 fathoms off the coast af Norway and named P. ahyssi. 



In 1883 Weismann 2 described a new species from the Gulf 

 of Naples which he named P. cidaritis. 



So far as I have been able to ascertaiu no further addition was 

 made to the list tili 1892, when the new species P. Jonesii was de- 

 scribed by US from Long Island Sound 3. 



This makes a list of at least 12 species, not including several 

 of doubtful affinities, which are as follows: 



1, P. muscoides Sars; 2. repens S. Wright; 3. minutus Allman; 

 4. sessilis S. Wright;. 5. palliatus S. Wright; 6. vestitus Allman; 

 7. serpens Allman; 8, linearis Alder; 9. multicornis Allman; 10. ahyssi 

 Sars; 11. ceV/anV/s Weismann; 12. Jonesii Hargitt & Osboru. 



Habitat. In reference fo the habit of the genus it may be 

 Said to be distinctively commensal. I have found no record of any 

 species of which a different character might be predicated. So far 

 as I have been able to ascertain, the followiug seems to be the general 

 ränge of habit so far as any bas been reported: 



P. muscoides^ attached to other Hydroids and tests of Ascidians. 

 P. repens^ attached to Sertularians and upon the back and legs of the 

 Spider crab ; P. minutus^ attached to the operculum of Turritella com- 

 munis \ P. sessilis, on Shells and occasionally on rocks; P. palli- 

 atus, attached to shells of hermit crab; P. vestitus, attached to old 

 Shells of Buccinum; P. serpens, chiefly on stems of other Hydroids; 

 P. linearis, on Shells of Turritella and other Gasteropods; P. cida- 

 ritis, on spines of Dorocidaris ; P. Jonesii, attached to abdomen and 

 pereiopods of spider crab. Concerning other species no records were 

 found. 



That some advantage accrues from this mode of life hardly admits 

 of doubt, though it may be difficult in each case to clearly per- 

 ceive in just what it may consist. In certain cases the commensalism 

 may approximate parasitism to a degree which is difficult to distin- 

 guish from it. Fewkes* has described an extremely interesting case of 

 what seems to be genuine parasitism among Hydroids in a species 

 which he named Hydrichthys mirus, from its habitat as a parasite 

 upon a fish of the genus Seriola [zonata, Cuv.). 



1 Forh. Vid. Selsk. Christiania 1874 pag. 91. 



2 Op. cit. 



3 Op. cit. 



^ Bull. Mus. Harvard Coli. Vol. 13 1888 ])ag. 224. 



