8 MOLLUSCA. 
even before my shadow could pass over them, they 
were aware of my presence, and endeavoured to 
withdraw into the shell. JI then cut off the eyes, 
with the thick cartilaginous tentacula in which 
they were lodged, but the animals still continued 
to be sensible of my near approach, while hanging 
in this mutilated and painful condition.” * Dr, 
Johnston records a manifestation of feeling, some- 
what analogous to this, in one of the common shell- 
fish of our sandy shores. “On a summer evening,” 
he remarks, “ I have observed the common Spout- 
fish (Solen siliqua), extended along the surface of 
the fine sand in which they burrow, enjoying, ap- 
parently, the calmness and mildness of the season, 
take alarm and instantaneously descend when I 
was yet distant several yards: and I can explain 
this and similar facts only on the supposition of 
the existence of a sense of touch feelingly alive to 
impressions impalpable to our grosser sense.” T 
The respiration is aquatic in most of the Mot- 
LuscA. ‘The breathing organs, in most cases, 
resemble in essential points the gills of fishes, con- 
sisting of a great number of leaves, often minutely 
subdivided. ‘They are chiefly formed of blood- 
vessels, covered with rows of vibrating cilia, by 
the constant motion of which, currents of water 
are perpetually hurled along the entire surface of 
the breathing organ, communicating oxygen, the 
vital principle, to the blood as they go, through 
the thin walls of the vessels. In many species, as 
the Bivalves, the gills form two large comb-like 
plates; in others they are arranged in the form of 
a feather; a beautiful tribe, known as naked-gilled, 
* Zool. Journ. iv. 172. 
¢ Introduction to Conchology, p. 199. 
