534 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



discs similarly give rise to the head and its ap- 

 pendages. 



In the developmental history of the Crustacea 

 there are two larval forms or stages which are very 

 widely distributed among the different orders ; the 

 appearance of these has been of very considerable 

 assistance in determining the real zoological position 

 of such forms as the barnacle and the parasitic Cope- 

 poda, which, when adult, have an appearance altogether 

 unlike that of Apus or Astacus (Fig. 219). 



Like many other larvae, these free-swimming forms 

 were, when first observed, thought to be distinct 

 animals, and received in consequence distinctive 

 names. The first is the stage known as that of the 

 Nan pi ins. In this the larva has an unsegmented 

 body and three pairs of appendages of which the two pos- 

 terior are biramose, a single median eye, and a distinct 

 digestive tract. In the lowest forms, the Phyllopoda, 

 this nauplius passes gradually into the adult stage, the 

 body becoming segmented, and fresh appendages ap- 

 pearing as the crustacean grows in size, and undergoes 

 its periodical ecdyses, or sheddings of the outer skin. 



Among the higher Crustacea (Malacostraca) the 

 larvae are hardly ever found freely swimming in the 

 Nauplius stage ; they more frequently make their 

 appearance at a more advanced period, or that which 

 is known as the Zoea. Here we have a cephalo- 

 thoracic shield, which is often, though not always, 

 provided with long spiniform processes, the longest of 

 which projects upwards from the middle of the back ; 

 the tail region is developed, but, like the hinder part 

 of the thorax, it is without the appendages that are 

 already developed in the anterior region of the body ; 

 lateral eyes are present in addition to a median one. 

 This Zoea stage is often succeeded by others, in which 

 certain characters are greatly exaggerated, or in which 

 there are presented arrangements which are permanent 



