238 



ANNELIDA AND ARTHROPODA: 



the basic diet, but some Cladocera are predaceous on 

 other branchiopods. 



Subclass OSTRACODA (Ostracods) 



Diagnosis: free-Hving with a bivalve carapace (see 

 Brachiopoda, Order Conchostraca) enclosing body, 

 but no more than three pairs of true thoracic append- 

 ages and these are not leaf-like; fresh-water and 

 marine, mostly bottom forms; size minute (Figure 

 14.12). 



Although not composed of calcium carbonate, the 

 ostracod's external skeleton is similar to a clam shell. 

 Most are marine, but many are common inhabitants 

 of fresh water. Their food consists primarily of bac- 

 teria, protozoans, molds, algae, and fine debris, and is 

 obtained by filtering the water. The filtering device, 

 which is formed by the mouth parts and front ap- 

 pendages, allows direct transfer of food to the mouth. 



Many produce their young from unfertilized eggs. 

 In some species males have never been found. 



Subclass COPEPODA (Copepods) 



Diagnosis: free-living or parasitic, without a cara- 

 pace; typically six pairs of thoracic limbs, not leaf- 

 like, but may be less in parasitic species; often three 

 simple eyes are fused into a single medial eye; most 

 are minute or microscopic; fresh-water and marine, 

 free-living to parasitic (Figure 14.12). 



Free-living copepods often are club- or pear- 

 shaped, but the parasitic species are quite different 

 and variable in appearance. Some of the parasites 

 are difficult to identify as arthropods, much less 

 copepods. Many of the parasites infest fish and are 

 called fish lice. 



The mouth parts of copepods are specialized for 

 raking, seizing, and scraping food from a surface, or 

 for filtering minute organisms from water. This 

 generally limits food items to protists and to organic 

 debris. 



Sexual reproduction is characteristic of this group. 

 After the eggs are fertilized they are retained by the 

 female, being kept on the abdomen in one or two egg 

 sacs. 



Subclass CIRRIPEDIA (Barnacles) 



Diagnosis: adults attached or parasitic; carapace 

 of attached species becomes part of a body enclosing 

 mantle, usually a fleshly mantle with calcareous 



plates; thoracic appendages simple, slender, and 

 bristly, not leaf-like, typically six pairs; abdomen 

 much reduced, vestigial; structures variously reduced 

 in parasites, ranging from lack of a calcareous shell 

 and reduction of appendages to a sac-like mass with- 

 out shell or appendages; fixed species with shelled 

 part of individual upon a stalk (gooseneck barnacles) 

 or without a stalk (acorn barnacles); regularly her- 

 maphroditic with free-swimming larvae; strictly ma- 

 rine (Figure 14.12). 



The nonparasitic barnacles start life much like 

 other marine arthropods. Nothing out of the ordinary 

 happens until a shrimp-like stage is reached. The 

 shrimp-like individuals seek a structure for attach- 

 ment, cement the backs of their heads to the struc- 

 ture, and grow their protective shell-like outer 

 covering. These shells may cause laymen to mistake 

 barnacles for mollusks. However, the fact that these 

 creatures are arthropods is evident if one examines 

 their jointed legs, which are used to collect minute 

 organisms for food. Free-living barnacles grow on 

 living and nonliving objects such as whales and sea- 

 shore rocks. Those that attach to ships can, of course, 

 cause damage. 



The parasitic barnacles look even less like arthro- 

 pods than do the free-living barnacles. Among this 

 group, which is found externally on mollusks and 

 crabs, are adults of species that are little more than a 

 sac-like reproductive organ. The reason zoologists 

 know that these animals are barnacles is that the 

 larval stages are similar to those of other barnacle 

 larvae. 



Subclass MALACOSTRACA (Malacostracans) 



Diagnosis: free-living to parasitic; body typically 

 of 19 segments, 5 head, 8 thorax (regularly covered 

 by a carapace), 6 (rarely 7) abdomen with append- 

 ages (except on rare seventh segment); mostly 

 marine, also freshwater, terrestrial, and parasitic 

 (Figures 14.13 and 14.14). 



This subclass includes the better known crus- 

 taceans. Although the Subclass Malacostraca con- 

 tains about twelve orders, only four are treated here. 

 Members of the other orders that might be found will 

 most likely be vaguely shrimp-like, hence for practical 

 purposes might be called "shrimps." The four orders 

 to be treated are the Isopoda, Amphipoda, Stoma- 

 topoda, and Decapoda. They can be distinguished as 

 follows: 



