464 LESLIE B. AREY 



'juvenile' mussel emerges from its cyst and enters upon an 

 independent existence. 



This investigation was undertaken to determine, first, the 

 nature and scope of the responsiveness of glochidia to a variety 

 of activating agents. Such a general survey of the sensory- 

 motor range preludes the more particular points of interest, 

 such as the nature of the stimuli leading to natural attachment 

 on the fish host and the induction of cyst formation. 



The experimentation was done at the United States Biological 

 Station, Fairport, Iowa, while holding a temporary appointment 

 from the Bureau of Fisheries. Acknowledgment is due the 

 Director, Mr. A. F. Shira, for the facilities of the station, and to 

 the Director, Superintendent H. L. Canfield, Dr. A. D. Howard, 

 and Dr. F. H. Reuling for material. It proved expedient to con- 

 centrate attention at first chiefly upon one form, Lampsilis luteola, 

 the Lake Pepin mucket; its glochidium is of large size, easily 

 obtainable over long periods, and belongs to the commoner, but 

 less known, hookless group. Later, comparative studies were 

 possible upon other species representing all types. 



II. THE GLOCHIDIUM 



A. Anatomy. There are two chief types of glochidium, dis- 

 tinguished by the presence or absence of marginal hooks on the 

 valves. It is customary, therefore, to speak of the hooked and 

 hookless glochidia. A third group, either lacking hooks or 

 having peculiar ones, is designated from its appearance when 

 closed as the axe-head type. 



1. Hooked glochidia. This type is characterized by bluntly 

 triangular and broadly hinged valves, each of which bears a 

 single, stout hook at its apex (figs. 1 and 2). The hooks possess 

 spines on their outer surfaces ; when the valves are approximated 

 the hooks are folded inward against the mantle, thus bringing 

 the spines in apposition. It is usually stated that special 'myo- 

 cytes' retract the hooks, but mutual contract, resultant from the 

 closure of the valves, is by itself quite adequate for this. If the 

 strong glochidial valves have closed on such a soft part as a fin or 

 a gill, it is evident that such an arrangement of hooks and spines 



