136 



going backwards over the intestine, and a sternal artery 

 which goes straight down to supply the ventral nerve cord. 



Contrast a typical Crab (Carcinus or Cancer) with a 

 Crayfish, and point out the principal Structural 

 Differences. 



The Crab has a broad cephalothorax, broadened by the 

 expansion of the branchiostegites at the sides. The abdomen 

 is greatly reduced and is permanently bent in under the 

 thorax and applied to its slightly-hollowed undersurface. 

 The abdominal appendages have also been reduced ; those 

 which are locomotor being absent, namely the last pair 

 (the paddle swimmerets) in both sexes and also the three 

 preceding pairs in the male. The male retains the first 

 two pairs of (copulatory) appendages only ; the female 

 has the first five pairs, and uses them for carrying the eggs. 



Mention the chief developmental Features of the Crustacea. 



The eggs or ova generally have much yolk, massed in the 

 centre, and consequently segmentation is partial and super- 

 ficial (meroblastic and peripheral). 



In most Crustaceans development involves one or more 

 pelagic larval forms (i.e., the life- history exhibits meta- 

 morphosis). Entomostraca have usually only one larval 

 form, a free-swimming Nauplius ; but in Cirripedes the 

 Nauplius is succeeded by a Cypris larva (see life-history 

 of Sacculina, page 127). The Malacostraca have higher 

 differentiated larval forms. The shrimp, Penaeus, has 

 three successive stages, a Nauplius, a Zoea, and a Mysis. 

 Crabs have two, a Zoea (with well- developed appendages, 

 a segmented abdomen, and a carapace with long spines) 

 and a Megalopa (crab-like but with the abdomen not yet 

 tucked under). The Lobster ( Homarus) is hatched out as 

 a Mysis with biramous thoracic limbs (used for swimming). 



The life - histories of the Decapod Crustacea show a 

 tendency to shorten metamorphosis in the higher forms 

 (e.g., the lobster begins at an advanced stage, the Mysis). 



In some Crustacea development is direct (e.g., most 

 water-fleas, Isopods, Amphipods, Astacus). Water-fleas 



