xii CRUSTACEA 177 



different also in form from tlie abdominal legs, 

 e.g. Oammarus, the Fresh-water Shrimp. 



Order 3. Isopoda. — In the Isopoda the head fuses with only 

 one thoracic segment, the body is flattened dorso- 

 ventrally, the legs are all of the same type, and the 

 gills are attached to the abdominal segments, e.g. 

 Asellus, the Water Slater ; Armadillidmm, the 

 Wood-louse ; Platyarthrus, the White Slater. 

 II. Lower Crustacea (Entomostraca). — These are small forms 

 with variable segmentation ; a median simple eye 

 may be present as well as compound eyes. 



Order. 1. BrancMopoda. — Small forms with leaf-like swim- 

 ming appendages, visually with simple and com- 

 pound eyes ; a large carapace is often present, e.g. 

 in JDaphnia, the Water Flea. 



Order 2. Gopepoda. — Small forms with no carapace and 

 usually about sixteen segments altogether ; compound 

 eyes absent, e.g. Cyclops. 



Order 3. Ostracoda. — Small forms usually of about eight 

 segments only, enclosed within a carapace having 

 a bivalve form, e.g. Gypris, Gandona. 



Order 4. Girripedia. — Much modified Crustacea, with a 

 body of a few segments surrounded by a fold of 

 skin, which secretes in most cases a number of calci- 

 fied plates that form a hard case found the body. 

 All are sedentary except for those few which are 

 parasitic, e.g. Balanus, the Acorn Barnacle ; Lepas, 

 the Goose Barnacle. 



Practical Work on Crustacea 



1. Study of Living Grustacea. — A live common shrimp, or better 

 still a small prawn, may be got straight from the sea or from some 

 marine biological station, and its habits watched in the sea-water 

 tank. When these Crustacea are kept in the tank, the bottom of 

 it should always be covered, at any rate jDartly, with sea sand, to 

 the depth of an inch or two, since the shrimp likes to biu?y itself 

 in this ; the water must be well aerated. Large anemones must 

 not inhabit the same tank, for they will devour their fellow- 

 lodgers. The Crustaceans can be fed on little morsels of fish. 

 They are often useful in a tank, for they will eat up any small 

 particles of food dropped by the other inhabitants. 



A very small crab and a small hermit crab may be kept vmder 

 the same conditions as the prawn, but care must be taken not to 

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