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-stzge. 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 Naup!ius-eye in addition to the stalked paired eyes. 

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

 poda pass through a Mysts-stege, 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 

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



