LARVAE. CARE OF YOUNG. 357 



Cypris, Apus, Branchipus and Leptodora (winter eggs) ; and 

 among the Malacostraca in Euphausia, Leucifer and Penaeus it 

 is astonishing what diverse forms are knit together by this three- 

 limbed larva. In all these the young enter on a free life as soon 

 as they are hatched, and the adult form is attained in a series 

 of moults by a more or less gradual metamorphosis. But in 

 other cases, as in the Cladocera, many Ostracods, Nebalia and 

 the ' Peracarida " the young are protected, in one way or 

 another, during their development, by the mother, and the 

 metamorphosis is greatly abbreviated. Nevertheless some 

 indication of the 3-limbed nauplius stage, and the throwing off 

 of a nauplius skin occur in all. 



In the Entomostraca with free swimming larvae the advance 

 to the adult state is by a series of gradual changes, but in the 

 Malacostraca a number of remarkable larval forms are found, 

 belonging to stages which succeed the nauplius, and adapted 

 to a pelagic existence (p. 448 ff). 



In most, though not in all, cases (and markedly not in the 

 Zoaea larva) the segments and appendages appear in order from 

 before backwards, the former being differentiated from a bud- 

 ding zone at the posterior end of the larva. 



Various contrivances are found among the Crustacea for the 

 protection of the eggs and of the young. In several groups the 

 batch of eggs is contained in a sack formed by the hardened 

 secretion of the oviduct. This may project freely as in the 

 Copepods, and some Euphausiidae, or lie in the space between 

 the dorsal shield and the body as in Cirripedes. In the Cladocera 

 many Ostracods and Estheria the eggs lie free in this space 

 and the whole of the development may occur in it. When 

 the slowly-developing winter eggs of the Cladocera are produced 

 the walls of the dorsal shield about them become thick and hard, 

 forming the saddle-like " ephippium" which being shed at the 

 next moult with the eggs contained in it, forms a bivalved case 

 for their protection. In the Apodidae two lobes of one appen- 

 dage become opposed to form the egg case on either side. In 

 many Malacostraca a brood pouch is formed as we have seen 

 by the modification of epipodites of some of the thoracic limbs 

 into oostegites, and between them and the ventral surface of 

 the mother the young are protected. In the Decapoda the eggs 

 are attached by a sticky secretion to the long setae on the 



