202 



INVERTEBRATA 



CHAP. 



its basal blade, whilst on the first antenna there is developed a 

 disc for fixation. A bivalve carapace now appears which replaces 

 the old three-cornered larval shield, the six pairs of thoracic legs 

 acquire swimming-hairs and take over the swimming function, the 

 compound eyes become functional, and the larva has now passed into 

 what is termed the Pupa stage. 



The pupa swims actively about for some time, but it takes no 

 food. It finally settles on a suitable spot, and attaches itself by the 

 disc on the first antenna in which a gland with a glutinous secretion 

 opens ; by the copious effusion of this secretion the larva is attached. 

 An ecdysis now takes place and, by the preponderant growth of the 

 skin of the ventral surface, the animal is rotated into a position in 

 which it may be described as standing on its head. The praeoral 



part of the body grows very much 

 in size and becomes the stalk, 

 and shelly plates, the scutum, 

 the tergum, and the carina, 

 are secreted by the folds of skin 

 which constitute the carapace ; 

 these calcareous plates replace 

 the chitinous shields of the 

 pupa. In this way the adult 

 Barnacle condition is attained 

 (Fig. 148). 



In the development of the 

 parasitic forms like Sacculina, 

 which draw nourishment from 

 the blood of their host through 

 root - like extensions of the 

 stalk, the Nauplius possesses 

 no mouth and is fed by the yolk in its endoderm, and the pupa 

 on fixation amputates the hinder part of the body. 



The Ostracoda also enter on their free-life as creatures with the 

 three appendages of the Nauplius, but the two flaps of skin con- 

 stituting the bivalve carapace of the adult are already developed, and 

 the second and third appendages consist of one branch only, the 

 outer branch or exopodite being lost. The passage into the adult 

 condition here is so gradual that one cannot speak of a critical moult. 

 In the development of the carapace and the unforked character of 

 the second and third appendages we have an anticipation of adult 

 characters (Fig. 149). 



When we turn our attention to the higher Crustacea we find that 

 the Mysidacea among the Schizopoda and all the Cumacea carry the 

 eggs in a brood pouch beneath the body of the mother, and from 

 these eggs young Crustacea hatch out with all the essential features 

 of the adult. But in the Euphausiadacea among the Schizopoda, and 

 in some genera at least of the tribe Penaeidea among Decapoda, 

 to which Lucifer belongs, the young leave the egg membrane as 



add 



FIG. 149. The Nauplius larva of Cypris ovum. 

 (After Clans.) 



aili!, adductor muscle of carapace ; at 1 , first 

 antenna ; aft, second antenna ; cer, cerebral ganglion ; 

 int, intestine ; TO?!, mandible ; oc, eye ; st, stomo- 

 daeum (stomach). 



