Decapoda — Mcicru va> 
usually greatly enlarged and very dissimilar in shape. The second Table-case 
legs are slender, and have the carpus, or “ wrist,” divided into No ' 9 - 
many small segments. The members of this family are very 
abundant in tropical seas, especially on coral reefs. Some of them 
produce a clicking noise by snapping the fingers of one of the 
chelae. 
In the family Palaemonidae the first two pairs of legs end in 
chelae, or pincers ; the second pair is larger than the first, and 
has the carpus, or “ wrist,” undivided. The antennules bear each 
three terminal filaments. To this family belong the common 
marine “ Prawns ” of British coasts and the “ River-Prawns ” that 
are abundant everywhere in fresh waters within the tropics. The 
great size reached by some of the latter is shown by the specimens 
of Palaemon carcinus from the East Indies and P. jamcticensis from 
Fig. 33. 
The common Prawn, Lcancler serratus, slightly reduced. [Table-case No. 9.] 
the West Indies. Attention may also be directed to a specimen 
of the common Prawn ( Lcancler serratus ) (Fig. 33) prepared by a 
special process so as to retain the translucency of the living animal. 
In the family Crangoniclae the pincers of the first pair of legs 
are imperfectly formed (sub-chelate) and much stronger than 
those of the second pair, which are very slender. The rostrum 
is usually short and flattened. To this family belong the common 
Shrimp ( Crangon vulgaris) and the large Arctic Shrimp ( Sclero - 
crangon boreas). 
The Tribe Astacidea (or Nephropsidea) includes the true Table-case 
Lobsters and Crayfishes. They may be recognised by having the No * 10 - 
first three pairs of legs chelate or pincer-like, and the first pair 
very large. 
The Lobsters constitute the family Homaridae, all the members 
of which inhabit the sea. The last thoracic sternite is firmly fixed 
