3l6 THE INVERTEBRATA 



which are uniramous, and the antennae and mandibles, which are 

 biramous and should each bear a gnathobasic process or spine 

 directed towards the mouth , though those of the mandibles are often not 

 developed at first. The antennal ganglia are as yet postoral (see p. 305). 

 The median eye is the only organ of vision. A pair of frontal organs 

 (p. 307) are present as papillae or filaments. There is a large labrum. 

 Fore, mid and hind guts can be recognized in the alimentary canal. 

 Antennal glands may be present. This larva is found in some members 

 of every class of the Crustacea, though among the Malacostraca only 

 certain primitive genera possess it, and in the Ostracoda it is modified 

 by having already at hatching a precociously developed bivalved cara- 

 pace. In every class, however, it is also often passed over, and becomes 

 an embryonic stage within the egg membrane or in a brood pouch, 

 the animal hatching at a later stage, such as the Metanauplius and 

 Zoaea mentioned below, or even almost as an adult. 



In the Branchiopoda and Ostracoda the Nauplius is transformed 

 gradually into the adult, adding somite after somite in order from 

 before backwards by budding in front of the telson, much as somites 

 are added to the trochosphere in the development of annelids, while 

 by degrees the other features Oi the adult develop. The early stages of 

 this process, which possess more somites than the Nauplius^ but have 

 not yet the adult form, are known as Metanauplii. The carapace is 

 often foreshadowed quite early by a dorsal shield, which later grows 

 out behind and at the sides to assume the form which it has in the 

 adult, and the appendages, at first mere buds, gradually take on their 

 final shapes. 



In most cases, however, the process just described is modified. 

 {a) It may make a sudden great advance at one moult. In the Cirri- 

 pedia the late Nauplius passes with a leap to the so-called Cypris 

 larva, which has many of the features of the adult : a similar leap takes 

 the copepod Metanauplius to the first ''Cyclops" stage (p. 334). 

 (b) Certain structures may be precociously developed. In those 

 of the Malacostraca which have Nauplii, the Metanauplius is followed 

 by stages, known as Zoaeae, in which the abdomen is well developed, 

 while the thorax, though it already possesses in front a few pairs of 

 biramous appendages, is still rudimentary in its hinder part. In 

 these larvae also the last pair of abdominal limbs usually appears, 

 or comes to functional development, before the others. Zoaeae, how- 

 ever, most often are not preceded by a free Nauplius but appear as the 

 first free stage (Fig. 277 A), (c) Temporary retrogression of certain 

 organs takes place during the development of some of the Mala- 

 costraca : this affects some of the thoracic limbs in certain Stomatopoda 

 and the prawn Sergestes, abdominal swimmerets and the antennule 

 in the prawn Penaeus. 



