32 A BIOLOGY OF CRUSTACEA 



and food particles are trapped in the midline between the legs, but 

 there is also a filter on the first thoracic leg and the second maxilla. 

 It looks as it Nebaliopsis has regained a maxillary filter after its 

 ancestors had lost it, but in reality the filter on the maxilla of 

 Nebaliopsis acts in combination with a filter on the first leg, and so 

 is quite different from the primitive malacostracan type. Further, 

 this filter is probably cleared and the food passed to the mouth by 

 the action of the mandibular palps. The sequence of the evolution 

 of Nebaliopsis, as visualised by Cannon, is as follows. Certain my sid- 

 like creatures took to burrowing in mud, gave up swimming and 

 developed a large carapace which protected their gills from the 

 clogging effect of their environment. The maxillary filter was lost 

 because it became clogged too quickly, and the animals fed on large 

 particles. They took to moving about in the mud by an oar-like 

 action of the antennules, which tended to suck water into the front 

 of the carapace. In this way an irregular water current was estab- 

 lished and later reinforced and regularised by the action of the 

 thoracic limbs, which became paddle-shaped. The increase in the 

 stream entering the front of the carapace would bring in more food 

 and many small particles which were eventually collected when a 

 new filter mechanism developed on the thoracic legs. This brings 

 us to a creature like Nebalia. It is then thought that some of these 

 creatures gave up living in mud and started to swim in the plank- 

 ton. This was accompanied by a redevelopment of a filter near to 

 the mouth, which was not now in danger of clogging, and a widen- 

 ing of the carapace, so that water was sucked in from all directions. 

 Later it stopped being drawn in from in front, and a stage similar 

 to Nebaliopsis was reached. 



Further work has shown that Nebaliopsis typica has an enormous 

 sac opening into its gut, and that the gut is modified for sucking. 

 It is now thought that N. typica is an egg sucker. The scarcity of 

 eggs in deep water makes it necessary for Nebaliopsis to take as 

 many eggs as possible when they are available — hence the enormous 

 sac for storage. This does not deny the possibility that the filter 

 may also be useful to tide the animal over periods when eggs are 

 not available. 



The scheme of evolution which has just been described gives an 

 idea of the complexity of the path that evolution has taken within 

 the Crustacea. The story becomes more complicated when we turn 

 to other groups of the Malacostraca. 



The evolutionary trend from the Mysidacea towards the Isopoda 



