ASSOCIATIONS WITH OTHER ANIMALS 109 



From living on the external surface to living on an internal 

 surface is but a short step, which leads to parasitism. We cannot 

 deal here with all the various parasites of Crustacea: the parasitic 

 Crustacea are dealt with in the next chapter. Some Crustacea seem 

 to be in the process of moving from the outside to the inside. 

 Ostracods have been found in the gill chambers of fresh water 

 crayfish in North America, and the copepod Tisbe elongata lives 

 in the gill chamber of the lobster. 



The water passages of sponges are often inhabited by a varietv 

 of small Crustacea, particularly shrimps of the family Alpheidae, 

 and some stenopids, such as Spongicola. which lives in pairs inside 

 the sponge known as Venus's Flower Basket. The shrimps enter the 

 sponge when they are young, but cannot escape after they have 

 grown. A pair of shrimps imorisoned in the skeleton of the sponge 

 is sometimes given as a wedding present in Japan, to symbolise 

 marriage until death. 



One of the most interesting internal commensals is the copepod 

 Ascidicola rosea, which lives in the oesophagus of ascidians or sea 

 squirts. The host feeds bv collecting small particles in mucus and 

 making a food string which passes down the oesonhagus under the 

 influence of ciliary action. The copepod feeds on particles which 

 it extracts from the food string. The legs bear long setae and the 

 abdomen has a spinv pad for holding on to the string. As the string 

 is continuously passing down the oesophagus the copepod has to 

 climb upwards at intervals to maintain its position. After each climb 

 it feeds for a while as it is transported downwards. The situation has 

 a parallel in someone walking up a downward travelling escalator. 

 The copepod does not spend all its life on the food string, but 

 occasionally wanders down into the host's stomach and deposits 

 some eggs which liberate nauplii, still enclosed in a thin membrane. 

 These nauplii travel through the host's gut and escape from their 

 membranes just as they are shot out with the exhalent stream of 

 the water current that the host continually pumps through itself. 

 The nauplii pass through six moults in six days and assume the 

 form of copepodids which seek a new host. 



Commensalism of a quite different type is found among certain 

 woodlice. Some, such as Kovmania and Schoblia, live in galleries 

 in termite nests, and others, such as Platvarthrus. live in ants' nests. 



Platyarthrus hoffmannseggi can be found in the nests of many 

 different species of ants, and it appears to be equally well tolerated 

 by all of them. The woodlouse feeds on the faeces of the ants, and 



