566 



ZOOLOGY 



SECT 



The most striking general character in the external features of 

 the Malacostraca is the limitation in the number j^egments. The 

 headHas~tFe same composition as in the Entomostraca, but the 



thorax is invariably formed 

 of eight segments, and, except 

 in the Phyllocarida, the 

 abdomen of six segments and 

 a telson. The limbs are strik- 

 ingly modified for the perform- 

 ance of various functions. 



The Phyllocarida are inter- 

 esting from the fact that 

 they are annectent or linking 

 forms between the Branchio- 

 poda and the Copepoda on 

 the one hand, and the higher 

 Crustacea, particularly the 

 Schizopoda and Decapoda, 

 on the other. The order 

 contains only three genera, 

 the commonest of which, 

 Nebalia (Fig. 464), is a little 

 shrimp-like marine Crusta- 

 cean about 6-8 mm. in 

 length. The body is divisible 

 into head, thorax, and abdo- 

 men, all having the normal 

 malacostracan number of - 

 segments except the abdo- 

 men, which is formed of 

 eight segments, the last bear-' 

 ing caudal styles structures 

 not found elsewhere in the sub- 

 class. There is a bivalved 

 cephalic carapace (s), closed 

 by an adductor muscle (sm) 

 and extending backwards to 

 the fourth abdominal seg- 

 ment : it is terminated in front 

 by a movable rostrum (r). 

 The eyes (a) are large, 

 raised on 



movably articulated stalks.^ 

 The antennules (a T ) and antenna (a 2 ) are large, the mandibles 

 (md.) have palps (mt), and the exopodite of the second maxilla 4 

 (mxt) has the form of a slender filament which acts as a " cleaning- 



FIG. 464. Nebalia geoffroyi, male. a. eye; 

 a\, antennule ; a%, antenna ; c, head ; brf. thoracic 

 feet ; d, intestine ; h. heart ; km, gizzard ; md. 

 mandible ; mt, mandibular palp ; mxt. exo- 

 podite of second maxilla ; PI p, pleopods ; 

 r. rostrum ; s, carapace ; sm, adductor muscle ; 

 t. testis; I VIII, thoracic segments. (From compound, and 

 Lang's Comparative Anatomy, after Glaus.) r n -, . . i 



