18 
aquarium. During the subsequent exper ment on the rate at which the sea- 
urchin destroys the mussels (see Table VII.) it was found that the method which 
the urchin employs when destroying the mussel is to chip away the mussel shell, 
beginning usually (if not always) at the posterior or siphon and to chew up 
the mussel shell bit by bit. In some cases the whole shell is chewed up in this 
fashion, in other cases the shell is only partially destroyed, the soft part of the 
mussel being eaten from the remaining part of the shell. If a-sea-urchin is 
disturbed in an attack on a mussel before it has been seriously damaged, the 
mussel shows a scratched and ragged edge. Many old mussel shells show that 
they have been attacked by sea-urchins. Evidence of such attacks is frequently 
found on the winter check marks. 
An attempt was made during the summer of 1919 to obtain some data 
regarding the rate of destruction of the mussels by their molluscan and echino- 
derm enemies. This attempt was only partially successful. The forms used 
in the experiments conducted were Purpura, the common whelk, the round 
whelk, starfish and sea-urchins. 
The experiments were conducted from August 28th to Sept. 19th. During 
this time no ‘‘hot waves’ occurred so that no difficulty was encountered in 
keeping the animals cool. In the experiments with the starfish and round 
whelks the animals were placed in large shallow wooden tanks in running sea- 
water. The sea-urchins were placed in a small aquarium in running sea-water 
to a depth just immersing the animals. The Purpura and whelks were placed 
in small aquaria in sea-water to a depth of from 5 to 7 cm. This water was 
renewed three or four times per day. The mussels used were given no special 
surface to which to attach except in the case of the experiment with the Purpura. 
In this experiment the mussels were allowed to attach to a piece of rock before 
beginning the experiments. No food other than the mussels was furnished any 
of the animals used in the various experiments. 
The result of the experiment with the Purpura and mussels is shown in 
Table V. 
During the experiment on the rate of destruction of mussels recorded in 
Table V. a number of the Purpura did not appear to feed for some days. When 
placed in the aquarium some of the Purpura immediately attached themselves 
to its sides (some not being covered by the water) and remained clinging thus 
apparently without any change in position until their removal from the aquarium 
September 8th. When discarded they appeared healthy. Those that have 
been considered as feeding on the mussels were moving on the rock to which 
the mussels were attached and were attacking them. Shortly after the removal 
of the other Purpura two of these on the rocks made their way to one side of the 
aquarium and both remained clinging there until September 13th, when one was 
knocked to the bottom. On September 14th both were again clinging to one 
side of the aquarium. By September 16th, however, it was found near the 
mussels. On September 19th one of the other Purpura, which had been feeding 
on the mussels, was found clinging to the side of the aquarium. In Table V., 
in estimating the number of Purpura feeding, no account has been. taken of these 
two latter movements since one tends to counterbalance the other and any 
resulting error is probably very slight. 
