538 THE ANIMAL KINGDOM. 



and in the quantity of secondary central yolk present. They are relat- 

 ively largest when development is delayed or unaccompanied by meta- 

 morphosis. There is often a vitelline membrane and sometimes a specially 

 secreted shell. The ova are rarely laid : they are attached by Cypris and 

 Argulus to foreign bodies ; deposited by the Stomatopoda in the burrows 

 which they inhabit ; left to float in a specially modified portion of the 

 shell the ephippium as ' winter' ova by the Cladocera. They are 

 generally carried about by the female, e. g. attached to the abdominal 

 feet as in Decapoda ; within the shell above the abdomen as in Cladocera ; 

 in a secreted ovisac, as by most Copepoda ; or in a sub-thoracic brood- 

 pouch by Arthrostraca. 



Development is rarely direct (Cladocera with the exception of Lepto- 

 dora and certain Land-crabs), and in those instances where it is apparently 

 direct or abbreviated (Arthrostraca), traces of metamorphosis are to be 

 found, e.g. the larval membrane shed in ovo when the two pairs of an- 

 tennae and the mandibles are formed in Oniscus and other Isopoda : or as 

 soon as the blastoderm is established, as in Amphipoda. The meta- 

 morphosis may be well-marked but intra-ovular, e.g. Mysis, Nebalia. 

 The majority of Entomostraca quit the egg as a Nauplius, an unseg- 

 mented larva provided with three pairs of appendages, the first uniramose, 

 the two others biramose, which correspond to the first and second an- 

 tennae and mandibles respectively of the adult. The second pair often 

 has a basal masticatory hook, and is innervated from a post-oral ganglion. 

 There is a median eye. Somites and appendages are formed by sub- 

 sequent growth accompanied by ecdyses. Among Malacostraca the 

 Schizopod Euphausia and Decapod Penaeus have a Nauplius-stage. The 

 latter passes through a Zoaea- and a Mysis-stage into the adult. The De- 

 capod Lucifer starts as a Meta-Nauplius with the rudiments of the two 

 pairs of maxillae and the first maxillipeds, in addition to the Nauplius 

 appendages : its ally Sergestes as a Protozoaea, in which all these limbs 

 are well-developed, and a cephalothoracic shield and unsegmented abdomen 

 are present as well. Most other Decapoda are hatched in a later stage 

 known as Zoaea, in which the second and third maxillipeds and a seg- 

 mented abdomen devoid of limbs are present. The thorax is short, not 

 segmented, but contains a ganglionic mass pierced by the sternal artery. 

 There is an azygos Nauplius-eye in addition to the stalked paired eyes. 

 The Zoaea like the Nauplius acquires secondary characters. Many Deca- 

 poda pass through a Mysts-stage, i. e. with biramose natatory feet on the 

 thorax, a stage in which the lobster (Homarus) is hatched. Development 

 is still more abbreviated and becomes almost direct in some forms, e.g. 

 Astacus. The larva of the Stomatopoda is known as A lima and Erichthus \ 

 of the Palinuridae (Decapoda) as Phyllosoma. The true Crabs (Decapoda 

 Brachyura) pass through a stage, Megalopa, resembling certain Hermit crabs. 



