ECTO-PARASITES AND COMMENSALS. 
423 
tli6y BiicouiitGr, wliicli is usually a luollusk, bGCOiiiuig scolicGs ov vgsi- 
cular worms amongst tliG couuGctivG tissuG, musclos, etc., but at a 
latGi- stagG occupying tliG digestive or respiratory passages which are 
in free communication with the exterior. 
The Ecto-Parasites are not nearly so numerous, the best known 
examples being the Aca?y, found so commonly upon the bodies of the 
terrestrial species, and the species of Aia.z^, which infest the Unionidee. 
The Conchophthirus lives within and probably subsists upon the 
body mucilage of various mollusks, both univalve and bivalve, while 
ChcctogastcT, Mutzui, and other forms live within the respiratory or 
mantle cavity of different species. 
Amongst the Ecto-Parasites must also be placed the Micrococcus 
conchivorus, a protophyte upon the shells and destroying its outer 
layers, a process in which the species of Batrachospermum, Chwto- 
phora, and other cryptogams assist. 
Fig. 738. — An Ectodermic Para- 
site from Hyalinia ccllaria (Miill.), 
Philodromus Ihnacmn Jenyns X 30. 
Fig. 739. — A Commen.sal, Mess- 
mate, or Mulualist, Epistylis coarc- 
iata Clap, and Lach.,a colony stock 
(highly magnified after Saville Kent). 
The Commensals or Mutualists are also often derived from the 
lower ranks of the animal world, and simply make use of the shell or 
body of the mollusk as an abode or place of fixation, and in such 
positions we may frequently find forests of Vorticella and other Infu- 
soria. That these colonies are not always harmless has been 
demonstrated by Mr. R. Standen, who has established the proba- 
bility that suitably-placed colonies of Epistylis or other urticating 
genera are often responsible for the scalarid and other monstrosi- 
ties of Planorbis spirorbis and doubtless other species. 
The Uses of the land and fresh- water mollusca in this country for 
food, ornament, medicine, or in the Arts are neither numerous nor 
important. Probably, however, the principal value of the mollusca 
to the human race generally is that they provide us directly and in- 
directly with abundance of food, and although the terrestrial mollusca 
