CRUSTACEANS 365 



adaptation between animals and their surroundings. In this par- 

 ticular instance it is very interesting to find that the trunk of a 

 crustacean develops in relation to the Nauplius in much the same 

 way that the trunk of a worm develops with reference to the 

 Trochosphere (see p. 359). For the hinder end of the Nauplius 

 is a region of rapid growth and elongation, and while this is going 

 on segment after segment comes into existence till the full number 

 are formed. And, as in the worm, the oldest segment is next 

 the head and the youngest next the tail. 



Higher Crustaceans (Malacostraca). — In Nebalia and other 

 Mud-Shrimps [Leptosiraca), which are in some respects interme- 

 diate between Higher and Lower 

 Crustaceans, the large shield which , "i 



grows back from the head to partly J _,^€ 2Sm^ /^^ 



cover the trunk affords a shelter to - — ^ — '^^^^^^^f^^^, 



the eggs until the time of hatching. ■'■' "^'^^^^^ ^^^M 



The development is direct, the young , ' X /^^^^^ 



mud-shrimps closely resembling the "" / ■ ''v^P^^^-vni 



adult form, except in size. '' - » 



The Sessile - Eyed Crustaceans Y,e" m.-Flm.i. sa„d-Hopper (g.,„».,„. 

 iArthrostraca), including: Sand-Hop- ««''^^"") ^''"yiig Egg' >" Brood-poud,, siightiy 



\ / <-> J- enlarged. i and 2, Antennules and antenna;; 



pers and Slaters, usually incubate ■; ■'• ''"d "■■■ ^^^^- 5=cond, and eighth thoracic 



. .11 1 1 limbs: rt(5, first abdominal limbs. 



their eggs m a brood -pouch on the 



under side of the thorax, the floor of the pouch being formed by 

 the growth of plates from the inner sides of the legs (fig. 888). 

 There is no free-swimming larva, development being direct. 



Stalk- Eyed Crustaceans {Thoracostraca) illustrate a number of 

 ways by which the eggs or young are protected. The little 

 Opossum Shrimp {Mysis), of which the development is direct, 

 possesses a brood-pouch, as its name indicates. This is of 

 similar character to the one found in Sand -Hoppers and their 

 kindred. In Lobsters, Crayfishes, Crabs, Prawns, Shrimps, and 

 the like, the eggs when laid are cemented to the numerous 

 bristles with which the abdominal appendages of the mother are 

 thickly studded. The material for this egg-glue is furnished by 

 glands which open on the under surface of the thorax. Every- 

 one must have noticed some creature of the kind " in berry ", i.e. 

 with numerous eggs firmly fixed to the lower side of the tail, 

 in which safe position the early part of the development is gone 

 through. 



