xi PHYLUM ARTHROPOD A 599 



of the adult. In Lepas the anterior region of the head grows out 

 into a peduncle. The pupal integument is then thrown off, the 

 paired eyes disappear, and the adult form is assumed. 



In Sacculina a still more extraordinary metamorphosis takes 

 place. The young is hatched as a nauplius, and passes into a 

 Cypris-stage. In this condition, after a brief free existence, it 

 attaches itself to the body of a young Crab, near the base of a seta. 

 The thorax with its appendages is thrown off, and the rest of the 

 body is converted into a rounded mass, from the anterior end of 

 which an arrow-like process is developed. This- perforates the 

 cuticle of the host, and, through the communication thus formed, 

 the whole body of the parasite passes into the interior of the Crab, 

 and becomes surrounded by a new cuticle, the old cuticle being 

 left empty on the outside of the Crab's body. The Sacculina now 

 sends out root-like processes, grows immensely, and, pressing upon 

 the body- wall of the Crab, causes atrophy of the tissues: this 

 allows the now greatly-swollen parasite to project on the exterior 

 as the tumour-like adult described above (p. 579), 



The embryo of Euphausia leaves the egg as a typical 

 free-swimming nauplius; this passes into what is called the 

 protozocua-stage, distinguished by the possession of an elongated, 

 unsegmented abdomen without appendages. After successive 

 moults, the rest of the appendages appear, and the adult form is 

 assumed. In Mysis (Fig. 459) the nauplius is maggot-like, 

 arid undergoes development in the brood-pouch, emerging in a 

 condition closely resembling the adult. 



The development of the Decapoda presents a very interesting 

 series of modifications. In two genera of prawns (Penceus and 

 Lucifer) the embryo leaves the egg as a nauplius, and passes by 

 successive moults through a protozoaea stage, a zocea-stage, with 

 segmented but limbless abdomen, and a mysis or schizopcd- stage 

 in which it resembles an adult Mysis, having exopodites to all 

 the thoracic limbs. 



In the Crabs the nauplius stage is passed through in the egg, 

 and the young is hatched in the form of a peculiarly modified 

 zosea (Fig. 475, A), with an immense cephalothorax produced into 

 spines, large stalked eyes, and a slender abdomen. This passes 

 by successive moults into the megalopa-stage (B), which resembles 

 an adult Macruran, having an extended abdomen with well- 

 developed pleopods. The megalopa passes by successive moults 

 into the adult form. 



In the Lobster (Homarus) both nauplius and zosea-stages are 

 passed through in the egg, and the embryo is hatched in the 

 mysis-stage with exopodites to all the thoracic limbs. In the 

 Rock-lobster (Palinurus) and its allies, the newly hatched young 

 is a strangely modified Mysis-form called a Glass-Crab or Phyllo- 



Q Q '2 



