Sub-Glass 2. Malacostraca. Order 6. Decapoda. 



221 



iairs* upon the skin. Such, free auditory hairs may occur 

 also in Crustacea possessing the vesicle, and may be present in other 

 regions {e.g., upon the abdomen). The Decapoda possess a very 

 strong gizzard, often with large, calcareous, masticatory teeth. In 

 depressions in its side walls there are often to be found two rounded 

 calcareous masses, which are absorbed before a moult (" crabs' eyes," 

 or gastroliths) . There is a large antennary gland, the " green 

 gland," opening by a small aperture in the base of the antenna. 



As the eggs leave the oviducts they are firmly fixed on to the 

 s-wimmerets of the female, which never possesses a marsupium, but 

 in spite of this almost always carries the eggs about ; not, how- 

 ever, the larvae, or these only for a short time. The young ones 

 almost always undergo a com- 

 plete metamorphosis. Only 

 in a small number does a 

 free-swimming nauplius 

 represent the first stage, e.g., 

 in the Prawn, Penceus {see 

 below), and in some allied 

 forms. The majority are 

 further developed before 

 hatching, having attained 

 the so-called zosea stage, t 

 in which condition the animal 

 moves by the appendages 

 which later form the m a x i 1- 

 1 i p e d s , and at this stage 

 are not connected with feed- 

 ing, but are solely natatory 

 in function. Swimming is 

 chiefiy effected by the exo- 

 jjods of these appendages. 

 The zoasa displays further, 

 the nauplius eye and lateral 

 eyes, the two pairs of antennee 

 and the three pairs of jawsj 

 the carapace also is present; 

 but the ambulatory legs, and 

 the swimmerets, have not yet 



appeared, or are only incipient, and the posterior part of the thorax, 

 and the abdomen, are not so pronounced as they become later. The forms 

 which hatch as nauplii, pass through the zosea stage later. In many 



Pig. 183. Zoaea of a Prawn enlarged, 

 •^a — 2/3 second and third maxillipeds. — After 

 Claus. 



* This is also the case in the Euphausidae. 



t Decapoda at this stage were formerly regarded as adxilt organisms, and described 

 under the generic name of Zojea; hence the name for these larvae. 



