134 



BIOLOGICAL LECTURES. 



surrounded by four elongate conical feelers, and is further under 

 the control of a set of muscles which effect its movements in 

 all directions about the long axis of the body, we see that 

 Bdellostoma is provided with a very sensitive and effective 

 organ of touch, and as such it makes continual use of it. 

 Each of me feelers is erectile ; that is to say, it may be laid 

 back against the side of the head or thrust out in any given 

 direction at the will of the creature (Fig. 4). Each feeler is 

 richly supplied with nerves and blood vessels, and has a slender 

 and flexible, though very tough filament running through it 

 to give it strength and to help in keeping its form. These 



FIG. 3. Ten representations of the positions taken by Bdellostoma at rest 

 (a, b, c, d, e, /,) and in motion (g, /i, /', /). a and l>, right- and left-handed coils. 

 c, with bent tail to act as support for body, i and /, single and double bow-knots, 

 into which Bdellostoma ties itself in order to draw its body through the knot for 

 the purpose of removing the thread cells or other foreign substances from the 

 surface of the body. 



sensitive feelers about the nostril give warning of the presence 

 of solid particles in the water which is being respired, for a 

 constant stream of water is drawn into the nose to pass into 

 the mouth through the hole in the roof of the palate and 

 thence onward to the gills. The genuine nasal nerve end-organ 

 is the simplest in structure of any presented to us by the 

 vertebrate series, and consist (Fig, 7) of seven semi-oval plates 

 of mucous membrane which hang from the roof of the nasal 

 cavity down into the stream of water as it turns downward 

 to pass into the mouth. These plates are bilaterally disposed 

 in sets of three, on either side of the median plate, which is 



