The viceral ganglion of Pecten, etc. 
19 
parable to that aroused by the starfish. It is well known that Pecten 
has a series of remarkable eyes arranged round the margin of the mantle 
and it is quite feasible to suppose that the starfish was observed and 
recognised by the animai as a cephalopod recognises a crab. To investi- 
gate this point a starfish was crushed into a pulp in a mortar and a pipette 
full of the fluid, forced at and between the mantle lobes of a Pecten. The 
result was a remarkable evidence of excitement foUowed usually by the 
animai swimming a short distance. This was not produced by the pipette 
when filled with sea water, nor by a concentrated plankton catch. Evi- 
dently the starfish is detected by the olfactory sense and hence the com- 
motion when one of these depredatory animals is placed in the vicinity 
of the mollusc. Experiments were now made with the osphradia and 
abdominal sense organ removed. The latter was first taken away by 
cutting out with a long pair of scissors. The individuals so treated were 
stimulated with a starfish or starfish pulp and the result was apparently 
exactly the same as that obtained when the normal animai was used. 
It was evident therefore that the abdominal sense organ played a very 
small part, if any, in the perception of Asterias. The experiments made 
with the osphradia removed were unfortunately not quite conclusive 
enough. These organs, as described above, Me in dose proximity to the 
visceral ganglion. An attempt was made to remove the sensory epithelium 
by scraping with a scalpel. Since however the osphradia are unpigmented 
it was not possible to see whether any remains of these organs were left. 
Moreover the Irritation always caused violent contractions of the ad- 
ductor muscle and the animals did not respond so actively after being 
placed once more in sea water. 
The individuals however, treated in this way stili appeared to react 
to the starfish pulp. Some reacted just as violently and perf ormed swimm- 
ing movements as vivaciously as the uninjured animals, whilst others 
merely closed the valves. On the whole, I believe the starfish pulp was 
recognised by the mantle edge, with its sensory tentacles and any action 
of the osphradium is very questionable. It must be pointed out that 
freshly caught Pecten are necessary for these experiments and not spe- 
cimens which may have been some time in aquarium tanks. It is hardly 
possible to remove the mantle edges from the sphere of action by severing 
the nerves, for the injury would be too great to expect reliable results. 
Since we may consider this movement caused by the starfish a protec- 
tive reaction, it would be rational to expect the olfactory organs to be 
situated on the mantle edges, where they would come quiekest under 
the influence of an approaching enemy. 
2* 
