162 INTRODUCTION TO ZOOLOGY chai'. 



" scaphognathite " attached to the second soft jaw or maxilla 

 (Fig. 97, i) \ this plate, by its movement, jerks the water out 

 of the gill chamber at its front margin, and so causes fresh 

 water to enter from behind. The blood is almost colourless. 

 The sexes are distinct : the fertilised eggs are 



t^on " ^^^^ ^'^^ ^'^^ carried by the female attached to 

 some of her swimmerets until they hatch ; they 

 are then protected in a brood-pouch made by the bending 

 forward of the abdomen. 



The larval 'prawns which hatch out are very unlike 

 the adult : they are little, soft, transparent creatures with 

 only rudiments of the thoracic walking legs, although all the 

 appendages in front of these are well developed ; the abdomen 

 also, though fully segmented, has at first no appendages ; those 

 on the sixth segment appear first, but the others do not 

 develop until all the thoracic legs are complete. These larvae 

 are known as Zooea larvae. 



As the immature larva develops, it goes 



EcdTOfs*"^ through a series of moults, and when it has 



acquired its final form and hard exo-skeleton the 



moult is still continued periodically, to allow for the further 



growth of the body. 



When the prawn is about to moult, its shell splits across 

 the back, just between the thorax and abdomen, and the 

 prawn gradually withdraws from the old shell first its 

 head, thorax, and anterior limbs, and then its abdomen. 

 The body is covered by the new exo-skeleton which has 

 been formed beneath the old, but this is still quite soft, and 

 whilst in this defenceless condition the animal has to remain 

 hidden in some sheltered corner for fear of enemies ; gradually 

 the exo-skeleton hardens and active life is resumed. 



At every moult, not only the external shell is shed, but 

 also the lining of the gullet and gizzard (the first part of the 

 stomach). This gizzard has on its inner walls thickenings of 

 the cuticle forming hard ridges, and also projecting teeth. 

 By means of special muscles these can be made to grind one 

 on another, and so the food swallowed is thoroughly 

 masticated. 



The compound stalked eyes must give a wide 



range of vision and the sight seems fairly keen.; 



the sense of tamh is specially located in the delicate antennae. 



