54 Papers from the Department of Marine Biology, 



nation of ridges and fringes of setae which occupy the posterior wall of 

 the stomach between the cardio-pyloric valve and the oesophagus. 

 These structures act as a filter to retain solid particles when water is 

 expelled, but owing to the fact that these are all of small size no definite 

 subcesophageal valve is present. 



The food organisms which are thus taken into the cardiac chamber 

 are all probably small enough to pass straight through the pyloric filter 

 and chamber into the mesenteron, where they are digested ; hence their 

 customary absence from the cardiac chamber. This organ, though 

 far too large for the requirements, has not been reduced in size since 

 Hapalocarcinus acquired its present habits. The pyloric filter is 

 preserved for the separation of occasional particles of larger size, but 

 these must be rare, since they were never observed in the stomach. 

 Almost certainly the crab must have the power of rejection of unsuit- 

 able pabulum. 



Porcellana, amongst the Decapoda Anomura, obtains its food by a 

 kind of net-fishing rather similar to that I have supposed occurs in 

 Hapalocarcinus. The appendages are provided with very thick borders 

 of hair, but particularly the third maxillipeds, on which they attain a 

 considerable length, greater than that in the gall crab. Gosse has 

 described the way in which Porcellana platycheles uses the third maxil- 

 lipeds by making alternate casting movements "exactly in the manner 

 of the fringed hand of a barnacle, of which both the organ and the 

 action strongly reminded me." I examined the appendages and 

 the contents of the stomach. In the former case little reduction is 

 shown (the protopodites of both maxillae are bilobed and the mandibles 

 possess a palp), but the thickness and strength of the fringe of setae, 

 developed even on the mandibular palp, show that all the appendages 

 are used for sifting and not for mastication. The stomach is usually 

 fairly full of food, generally in small unrecognizable fragments, occa- 

 sionally larger pieces of algae. The proportion of planktonic organisms 

 is not great and this is no doubt accounted for by the habitat of Por- 

 cellana under stones in littoral situations where the water is more usu- 

 ally muddy than clear, and organic debris abounds. So that while a 

 similar method of obtaining the food is practised in both forms the 

 nature of the food differs. In Hapalocarcinus it is much more minute 

 and entirely in the form of living organisms. 



It occurred to me that other of the higher Crustacea which had 

 adopted a sedentary existence might have developed a similar dietary 

 and alimentary apparatus. Pinnotheres lives within the depths of 

 the mantle cavity of a mollusc or the tube of a worm. Its tinj^ chelae, 

 like those of Hapalocarcinus, show that it is in no sense a predatory 

 animal. It might be expected that it would live on minute plankton 

 like its host. If the size of the animal is to be taken into consideration, 

 some species of this genus are not very much larger than the gall- 



