118 
agency of some encrusting Corals, Polyzoa, and a peculiar species of Prodiic- 
tidm called Etheridgella complectens, milii, sp. The enlargement does not 
generally appear to he effected so much hy a perforation of and hurrowing 
into the stems, as by irritation set up by the foreign organism, and a conse- 
quent effort on the part of the Crinoid to secrete calcareous matter and invest 
the pest. 
A similar enlargement and irregularity of growth is also very marked 
in stems of the Jurassic genera Apiocrinus, and Hillericrinus. The former 
was also studied by Mr. Pofe, and both have been by I)r. L. von Graff,^ who 
ascribes themischief to 3Igzostoma, a genus of Anarthropoda. No doubt some 
of the appearances produced in Carboniferous Crinoids are also due to the 
action of a Myzostomid, for I have figured^ a column in which the occupant 
of the burrow appeared to be an Annelid. 
Dr. P. II. Carpenter says^ tliat the external parasites of living Crinoids 
are many and various, such as “ small bivalves, Sertularian Tlydroids, Polyzoa, 
tube-worms, and corals,” but chief amongst them is 3Iyzostoma, Graff. The 
latter attaches itself to the stalk, disc, and arms, usually causing an abnormal 
growth of tbe calcareous tissue so as to form a cyst. He adds, “ I have 
never met with any distortion of the stem [of a living Crinoid] which 
could be considered as resulting from the action of a parasite, and it is 
therefore curious that abnormal growths in the stems of fossil Crinoids 
should have attracted the attention of so many Paleontologists.” 
The portion of column rejn’esented in PI. XYIII, Pig. 3, is unfortu- 
nately broken short off about midway on the swelling, but not without 
showing that the enlargement was accompanied by the usual depression of 
the surface, and apparent perforation. The former breaks the continuity 
of six ossicles of the column from its fracture upwards, but this is possibly 
only half the original depression. The swelling or enlargement is above the 
latter, the column at its upper end having a diameter of three-eighths 
of an inch, whilst at the most inflated part it is five-eighths. This specimen 
was obtained by Mr. C. Cullen at Ellalong, near East Maitland. A 
second swollen stem has been presented by the Chief Surveyor, Mr. Twynam, 
from the parish of St. Aubin, Pouchel Brook District. A section has 
distinctly revealed the cavity, which seems to have given the penetrating 
* Ueber eiuige Deformitiiten an fossilen Crinoiden. Palaeontographica, 1SS5, XXXI, p. 18o, t. 16. 
^ Loc. cit., t. 4, f. 18 ami 19. 
3 Challenger Report, Zool. XII, 1884. Report on the Crinoiclea, pp. 133 and 133. 
