CRUSTACEA 353 



lab rum. 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 Mala- 

 costraca only certain primitive genera possess it, and in the Ostracoda 

 it is modified by having already at hatching a precociously developed 

 bivalved carapace. 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 Meta- 

 nauplius 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 of 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 makes a sudden great advance at one moult. In the Cirripedia 

 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. 373) and those 

 of Malacostraca to the Zoaea. {b) Certain structures may be pre- 

 cociously 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 develop- 

 ment, before the others. Zoaeae, however, most often are not 

 preceded by a free Nauplius but appear as the first free stage (Fig. 

 291 A), (c) Temporary retrogression of certain organs takes place 

 during the development of some of the Malacostraca: this affects some 

 of the thoracic limbs in certain Stomatopoda and the prawn Sergestes, 

 abdominal swimmerets and the antennule in the prawn Penaeus. 



Class BRANCHIOPODA 



Free Crustacea with compound eyes ; usually a carapace ; the mandi- 

 bular palp very rarely present and then as a minute vestige; and at 

 least four, usually more, pairs of trunk limbs, which are in most cases 

 broad, lobed, and fringed on the inner edge with bristles. 



