114 ANIMAL PAKASITES AND MESSMATES. 



they form around the gills a kind of living moss, which 

 at last kills the fish. 



There are different leeches which inhabit invertebrate 

 animals. Bang mentions a little creature of this kind 

 in Senegal, living as a parasite upon the respiratory 

 apparatus of an anodont. Gay discovered in Chili one 

 of the Hirudinidas in the pulmonary sac of an Auricula, 

 and another on the branchiae of a crab (Branchiobdella 

 Chilensis). Mons. Blanchard has noticed a malaco- 

 bdella in the branchiae of the Venus exoleta ; and it was 

 known in the last century that the Mya truncata of our 

 coast also lodges a malaeobdella which lies always under 

 the foot of the animal. This is the hirudinean of which 

 we have spoken above, which is allied transitionally to 

 the trematoda. 



Together with the Hirudinidse, we find very small 

 worms, transparent, bristling with daggers and spikes of 

 every form, which are found everywhere in fresh water. 

 They are known by the name < of Nais. They are so 

 completely transparent that we can see the action of all 

 their organs through the substance of the skin. They 

 have been the subject of several remarkable works. 



They live freely among the leaves of Lemna and 

 other aquatic plants; but there is one species much 

 more restricted in their habitat than the others ; these 

 seek assistance from the Lemnese, and live at their ex- 

 pense. It is because of this kind, of which the genus 

 Chcetogaster has been formed, that we mention them 

 here. Their long bristles are veritable halberds, which 

 they employ with astonishing skill, both in attack and 

 defence. 



Among free parasites are found many very important 



