CRUSTACEANS 



413 



often found swimming in large shoals, and superficially resembling 

 ordinary shrimps, though of much smaller size. They differ, how- 

 ever, from these in many important particulars, among which 

 may be mentioned the delicate nature of the shield covering the 

 cephalo-thorax, to which at least one thoracic segment is not united. 

 There are, further, eight pairs of closely similar thoracic legs, pro- 

 vided both with outer and inner branches. The commonest 

 British genus is My sis (fig. 253), which so closely resembles a 



ANTENNA' 



Thoracic" 



LIMBS 



Fig. 253. — Opossum-Shrimp [Mysis], enlarged 



Stage in the development of the Lobster that this is spoken of as 

 the " Mysis stage ". Closed auditory sacs are present in its tail. 



3. Locust-Shrimps (Stomatopoda) are much larger (fig. 250) than 

 the members of the last sub-order, and are commonest in tropical 

 seas, where they may attain a length of as much as 8 inches or 

 more. The cephalo-thorax is small, for not only does it not include 

 the last three thoracic segments, but it does not have to shelter the 

 gills, as these are attached to the appendages of the broad well- 

 developed abdomen. The thoracic appendages are remarkable, for 

 the first five pairs of them are modified into foot-jaws, of which the 

 second are extremely large and modified as seizing-limbs (fig. 250). 

 So strikingly do they resemble the first pair of legs of the Praying 

 Mantis (see p. 381), which have a similar function, that these 

 Crustacea are often known as " Mantis-Shrimps". The last three 

 pairs of thoracic appendages are leg-like, and possessed of both 

 outer and inner branches. Two species are not uncommon in the 

 Channel Islands {Squilla Desmaresti and Squilla mantis). Like 



