584 



ZOOLOGY 



SECT. 



brief free existence, it attaches itself to the body of a young Crab, 

 near the base of a seta, by means of its antennae. The thorax with 

 its appendages is thrown off, and the rest of the body is converted 

 into a rounded mass of cells. The antennae perforate the cuticle 

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

 mass of cells passes into the interior of the Crab, and is carried by the 

 movement of the blood until it comes to rest in the thorax. 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. 565). 



FIG. 482. Larvse of Crabs. A. Zocea-stage of Maia ; S, Megalopa-stage of Fortunus. 

 h. heart ; a^- - a e , abdominal segments ; 1, antennule ; 2, antenna ; I VIII, thoracic appen- 

 dages. (From Lang's Comparative Anatomy, after Claus.) 



The embryo of Euphausia leaves the egg as a typical free-swim- 

 ming nauplius ; this passes into what is called the protozocea-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. 466) the nauplius is maggot-like, and undergoes develop- 

 ment 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 protozosea stage, a zocea-stage, with 

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



