298 



COMPARATIVE ANATOMY 



CHAP. 



Family 2. Lpphogastridse. 



With branchial tufts on the thoracic feet ; last 5 thoracic segments as in the 

 Mysidce. Lophogaster. 



Family 3. Euphausidae. 



With branchial tufts on the thoracic feet. Only the last thoracic segment is 

 free under the dorsal shield. Euphausia, Thysanopod.a. 



\UL 



FIG. 202. Astacus fluviatilis, male, from the side, cth, Cephalo-thorax ; ab, abdomen ; kd, 

 branchiostegite of the cephalo-thoracic shield ; XIV, first, XIX, last but one abdominal segment ; 

 r, rostrum ; 0,1-0%, 1st and 2d antennae ; 8, third foot-jaw or maxillipede ; 9, chelate foot ; 10, 

 11, 12, 13, the 4 remaining ambulatory feet ; 19, the pleopoda of the 6th abdominal segment, 

 which with the telson or terminal segment form the caudal fin (after Huxley). 



Order 4. Decapoda. 



Cephalo-thoracic shield large, generally firm and hard, calcareous, covering the 

 whole thorax, and fused with the dorsal integument of all the thoracic segments. 

 Exopodite of the 2d maxilla forms a vibratile plate which regulates the streaming of 

 the water in the branchial cavity. The 3 anterior pairs of thoracic feet developed as 

 foot-jaws or maxillipedes, the 5 posterior, some of which are armed with pincers, as 

 ambulatory feet (hence " Decapoda "). In the adult these ambulatory feet consist 

 only of protopodite and endopodite, while the exopodite is almost always wanting ; 

 auditory organs on the basal joints of the inner antennse. Development director 

 with metamorphosis. In the latter case a Nauplius is seldom (Carididce) hatched 

 from the egg ; the larvae hatched are generally further developed, and pass through a 

 Zocea and a Mysis stage. This order is very rich in forms. 



Sub-Order 1. Macrura. 



With well-developed abdomen, which is at least as long as the cephalo-thorax. 

 Mostly with the full number of pleopoda, the last pair of which forms with the 

 telson a powerful caudal fin. Carididaz (shrimps) : Penceus, Palcemon, Crangon, Pon- 

 tonia, Alpheus, Sergestes, Lucifer, in the sea. Astacidce : Astacus flumatilis (Cray- 

 fish, freshwater ; Figs. 202 and 203), Homarus (lobster), Nephrops, Callianassa, Gebia, 

 marine. Palinuridce : Palinurus, Scyllarus, in the sea. 



Sub-Order 2. Anomura (this division cannot be sharply demarcated). 



Abdomen moderately large ; caudal fin mostly reduced. The hindermost pair of 



ambulatory feet or the two posterior pairs reduced. Third foot -jaws limb-like. 



Paguridce, hermit-crabs, marine, in empty shells of Molluscs ; abdomen soft skinned, 



asymmetrical, with degenerated pleopoda serving as clinging organs. Pagurus, Eupa- 



