IMMATURE SPECIMENS OF PENELLA FILOSA. 



M. T. THOMPSON. 



The young stages of all parasitic Crustacea are of interest, and 

 this is especially true with respect to the highly modified forms. 

 Hence I was pleased to find among a number of Penclla filosa 

 loaned me by Dr. J. S. Kingsley, a few immature specimens. Pc- 

 nellajilosa is the commonest species of Penclla on our coast and 

 in all probability is unhesitatingly to be identified with the form 

 described from the sword-fish by Linnaeus in 1758 as Pcnnatitla 

 filosa under the impression that it was a Sea Pen, an error first 

 corrected by Pallas eight years later. This species infests the 

 sun-fish - - Orthagoriscus - - as well as the sword-fish - - Xiplrias 

 -and the double host has suggested to some investigators that 

 the name " filosa " covered an assemblage of ill-described Penellas 

 rather than characterizing a distinct species. This view, how- 

 ever, does not seem well taken. With respect to the forms 

 usually called by the name "filosa," material from either the 

 sun-fish or the sword-fish presents no constant peculiarities by 

 which the source may be determined. My immature specimens 

 were taken from sword-fish. 



The adult Penclla filosa frequently attains a length of i 30 mm. 

 and may be over 20 cm. long. Of this, almost one half, the 

 round "head" and slender cylindrical " prae-thorax," is buried 

 in the tissues of the host. Only the thicker "post-thorax " and 

 brush-like abdomen trail free in the sea-water, furnishing a locus 

 for a dense growth of hydroids and occasionally barnacles, Con- 

 choderma. The abdomen is less than one third of the exposed 

 part of the animal and owes its brush-like appearance to from 

 twenty to thirty pairs of lateral appendages each of which is sub- 

 divided into numerous thread-like branches. The trailing egg- 

 sacs are filiform and not infrequently measure 8 or 10 cm. in 

 length. The " head " retains minute, elongate, first, and stouter, 

 chelate, second antennae on the dorsal surface. Mouth parts 

 are absent and the ventral and anterior surfaces of the " head " 



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