The Swan-Mussel and its Anatomy. 73 



months — from September to April inclusively. And now comes 

 a very curious part in their history : the young animals after 

 exclusion are found to be parasitic upon fish, to the fins and 

 gills of which they attach themselves by their barbed hooks. 

 So different is the form of the embryo mussel from its parent 

 that many authors have regarded these young ones as para- 

 sites in the adult animal • to these supposed parasites the 

 name of Glochidia* was given by Rathke in 1797. How 

 long these little creatures consider it necessary for their well- 

 being to remain with closed valves upon the fin of an un- 

 fortunate fish, is a point which, I believe, has not yet been 

 determined, and it is one, the solution of which I recommend 

 to the investigations and patience of all Intellectual observers. 

 Whether again this parasitism is absolutely essential to the 

 growth of the young animal, or whether it can develope itself 

 without such attachment, is a question which can only be 

 answered by close observation and by experiment. I may 

 refer the reader, who takes an interest in this matter, to my 

 own paper " On the Fry of Anodonta Cygnea," in the Quar- 

 terly Journal of Microscopic Science (July, 1862), for the first 

 record of the parasitic nature of these little molluscs. 



There is a curious aquatic mite which is invariably found 

 parasitic within the mantle and uuder the gills of the swan- 

 mussel. These acari, which sometimes exist in great numbers 

 within the shell, deposit their eggs within the folds of the 

 inner branchias principally, though the ova and larva3 may 

 also be seen attached to the interior surface of the exterior 

 gills. Mr. Grwyn Jeffreys, who identifies this mite with the 

 At ax ypsilophora of Buntz, says that it is " so tenacious of 

 life that after the host has been put into boiling water and 

 killed, these little parasites survive and crawl about as if 

 nothing had happened ! " 



We must only just allude to the nervous system of the 

 fresh-water mussel, which consists of at least three well-marked 

 pairs of ganglions with connecting and branching nerves. 

 (1) The labial ganglia are situated on the sides of the mouth ; 

 they are oblong and bilobed, and connected by a nervous 

 filament. (2) The pedal or foot-ganglia are placed between 

 the visceral mass and the foot; they are larger than the 

 first-mentioned ganglia and more distinctly separated into two 

 parts ; this pair is connected by nerves with the labial 

 ganglia. (3) The posterior ganglia are situated on the lower 

 surface, a little below the posterior adductor muscle, near 

 the anal orifice. 



" All Lamellibranchs possess at least three pairs of prin- 



. .. * Crr. 7A,a>x'Sj " a point." 



