Introduction to Animal Morphology. 351 



Artemia lives in brine pools and salt lakes. Nebalia has a 

 •secondary binocular larval stage. 



Sub-class 5. Trilobitse {WalcK) — palaeozoic, shield-like 

 forms of 6-20 somites, divided into three lobes by two longi- 

 tudinal grooves. Cephalothorax semicircular, with two 

 lateral compound eyes or none ; between these is the 

 glabella ; outside them the gence; below the glabella is the 

 labium or hyposiome ; the middle lobe of the abdomen is 

 called rachis, the laterals />/f«r<£ ; behind the abdominal seg- 

 ments is a post-abdomen whose rings are united, more or 

 less, into 3. pygidium ; the limbs are unknown; between the 

 •eye and glabella is a depressed area, the fixed cheek, and 

 traces of the several component somites of the cephalothorax 

 are often present as imperfect transverse sutures or grooves. 

 The young was Naupliiform with unsegmented body. About 

 ^00 species are known, mostly Silurian. 



CHAPTER XLII. 



CRUSTACEA. 



Sub-class 6. Poecilopoda [Latreille] — heteronomously 

 segmented Crustacea, covered by two hard dorsal 

 shields, one behind the other, the front one being 

 convex forwards ; eyes two, sessile, compound, far 

 apart, on the front shield ; cornea unfacetted ; six 

 pair of limbs around the mouth ambulatory, ending 

 in pincers in the females,* and with their basal joints 

 spinous, and acting as jaws ; the five posterior pair 

 are lamellar, gill-bearing and natatory. Two ocelli lie 

 between the compound eyes ; the pharyngeal nerve- 

 ring is wide, with transverse commissures ; the simple 



* In the males the first (Limulus Polyphemus), or first and second pair 

 ^L. IMoluccanus), have but one claw at the end. 



