THE LEPTOSTRACA 



•59 



Kkmaijks on Habits, etc. 



All the Leptostiaca are marine. Most of the species occur in 

 shallow water or at moderate depths, but A^ekdiopsis l)elongs to the 

 " bathypelagic " fauna at 

 depths exceeding 1000 

 fathoms. Many have an 

 extremely wide distiibution, 

 the common European A77/((7/rt 

 bipes, for instance, ranging 

 from Greenland to Chile and 

 Japan. The species named 

 appears to be very resistant 

 to unfavourable conditions, 

 thriving in water which is 

 foul with decaying matter. 

 Locomotion is effected by 

 powerful strokes of the an- 

 terior four pairs of pleopods. The thoracic limbs serve the purpose 

 of respiration, and by their rhythmic movements produce a current 

 of water which brings food-particles to the mouth. The water is 

 drawn in from behind and expelled in a stream below the rostral 

 plate. 



The largest of existing Leptostraca is Nebaliopsis ii/pica, Sars, 

 Avhich reaches a length of about 40 mm. The other species are 

 from 4 to 12 mm. in leuirth. 



Embryo of Ndialia, .just liatclii'd. a', antennule ; 

 a", antenna; /, caiulal fuica ; i/n/, mandible; inj;', 

 maxillula ; mx", maxilla; plA, first jjleopod ; pl.6, 

 sixth pleopod ; tli.'*"', last tlioracic appendage. 

 (After Clans.) 



r 



Palaeontology. 



Certain fossil Crustacea, generally grouped together as a family, 

 Ceratiocaridae, are believed to be more or less closely allied to 

 the existing Leptostraca. The various genera, ranging from the 

 Cambrian to the Triassic epochs, differ considerably among them- 

 selves, but the more typical forms, such as Ceratiocaris (Fig. 93) and 

 IL/menocaris, resemble Nehalia in general form, having a bivalve 

 carapace, sometimes with an articulated rostral plate, and an ex- 

 tended abdomen. Nothing is definitely known of the appendages. 

 A pair of serrated plates observed within the outline of the shell 

 in some instances have been described as "gastric teeth," but 

 may possibly be the mandibles {m). Some of the fossils are of 

 great size, Ceratiocaris, from Ordovician and Silurian rocks, reaching 

 a length of two feet. 



The chief difference which can be ol)served between the fossils 

 and the living representatives of the Leptostraca is that, in the 

 former, the terminal appendages of the abdomen are always more 



