METHODS OF ATTACK AND DEFENCE in 



Bauer has noticed an absolutely similar case in an animal 

 belonging to quite a different group, i.e. Pecten jacobaus. 

 The remarkable swimming motions of this bivalve are only 

 induced by the chemical stimulus of the secretion of the 

 skin of the starfish. A similar phenomenon with regard to 

 this secretion has been noted above (p. 107) when we were 

 dealing with the function of Echinoid pedicellarise. 



While watching a starfish attempting to overpower an 

 individual of Murex hrandaris Bauer came to the conclusion 

 that the strongly developed spines on this Gasteropod are of 

 the nature of a protective adaptation, the starfish being 

 prevented by the spines from enveloping the shell with its 

 stomach. Bauer believes the strange forms of other 

 molluscs such as Murex trunculus, Typhis tetrapterm, 

 Trochon carinatus may be due to a similar cause. Again, the 

 young of Australium rugosum have a strong resemblance to 

 a cog-wheel, and so in spite of their small size are still pro- 

 tected from the attacks of the starfish. In the adult animals 

 the spines become less and less well developed, so that their 

 protective value is doubtful. On the other hand, the surface 

 of the shell becomes covered with numerous conchoidal 

 scales, the shell coming to resemble closely a rasp or file. 



Bauer also develops an interesting theory with regard 

 to the relations between Gasteropods and hermit crabs as 

 suggested by a conflict in an aquarium between a hermit 

 crab and a snail {Murex hrandaris). The hermit crab had 

 fastened on to the Murex and, while holding fast to the 

 spines, endeavoured to introduce its pincers into the shell 

 opening. When, after repeated attempts, the crab had at 

 length succeeded in jamming its pincer a little way between 

 the edge of the shell and the glass of the aquarium, the 

 snail raised itself a little and as the crab, taking advantage 

 of the opportunity, advanced, the snail quickly jumped 

 back against the side of the vessel. This movement was 

 repeated at short intervals, when the crab, either through 

 the shaking or the pressure on its chela, temporarily desisted 

 from its efforts. Shortly afterwards the battle began anew. 

 The crab had again got its pincer under the shell and the 



