200 CRUSTACEA. PAET in. 



first period of their lives, whence their name, ' Opossum 

 Shrimps' : the young are crowded in this pouch, and 

 acquire their adult form before they come into the water. 

 The circulation of the white blood of the Mysis was 

 discovered by Mr. Thompson : the pulsations of the heart 

 are so rapid that they resemble vibrations. There are 

 many species of these small shrimps. 



The genus Lucifer is one of the most singular of the 

 crustaceans from its almost linear form (fig. 149), the 

 excessive length of the anterior part of the head, the 

 extreme shortness of the thorax, the smallness of the 



Fig. 149. Lucifer, a stomapod crustacean. 



carapace or shell, and the great development of the tail, 

 which is more than three times as long as the thorax. 

 The thin eye-stalks, which are of exaggerated length, 

 extend at right angles from the top of the long cylindri- 

 cal part of the head, and terminate in large, staring, 

 dark-coloured eyeballs covered with a multitude of fa- 

 cettes. The two pairs of antennae are placed between and 

 below the eye-stalks. The undermost pair, which are 

 the shortest, have a little lamellar appendage at their 

 base : in some Lucifers, when viewed in front, it looks 

 like a cross. The salient mouth is placed at the base of 

 the long organ that carries the eye-stems. It has strong 

 toothed mandibles, two pairs of jaws with plates attached 

 to each jaw, and three pairs of foot-jaws. The tail is 

 very narrow, consisting as usual of seven rings movable 

 on one another ; but they are quite abnormal, for each of 

 the rings is at least as long as the thorax ; the last has 



