568 



ZOOLOGY 



SECT'. 



branchise ; there are stalked eyes and a fan-like tail-fin formed of 



the telson and the expanded nropods. 



The Mysidacea (Fig. 466) are 

 small, transparent, shrimp - like 

 forms, mostly from 2 6 mm. in 

 length. They agree with the Cray- 

 fish in the general form of the body, 

 in the union of the head and thorax, 

 in the presence of a carapace which 

 leaves some of the posterior thoracic 

 segments free and in the number 

 both of segments and appendages, 

 but present several interesting 

 characters indicating a lower grade 

 of organisation. One of the most 

 notable of these is the absence of 

 differentiation in the thoracic appen- 

 dages, which, though they have a 

 leg-like and not a leaf-like form, are 

 all alike, none of them being modi- 

 fied into maxillipedes, except to a 

 very slight degree in some forms. 

 Moreover, the legs all possess exopo- 

 dites (ex), thus retaining the primi- 

 tive biramous or " split-footed ' 

 form which is lost in the Decapoda. 

 The first five pleopods are large in 

 the male, small in the female : the 

 sixth is a uropod, i.e., assists the 

 telson in the formation of the charac- 

 teristic malacostracan tail-fin : there 

 is no trace of the entomostracan 

 caudal styles. 



The Cumacea are also a very 

 small group : Diaslylis (Fig. 467) is 

 a good example. They are little 

 shrimp-like animals, differing from 

 all the Malacostraca previously con- 

 sidered in having poorly developed 

 sessile eyes, sometimes fused to- 

 gether, and in some genera altogether 

 \ , ..?; V' r absent. The carapace (cth) is so 

 small as to leave the five posterior 

 segments (tli IV 7777) uncovered. 



Kio. 407. Diastylis Stygia. o,, ;m- rpi r> , , ^ir* nf flinr-ipip limlist 



tennule; .j , antenna; afc., ab. 7 , ab- tw pairs OI tnoraCIC limbb 



dominai segments; cth. cephalothorax; are maxillipedes. the last six, less : 



en, endopodite; ex, exopodite ; p.], ?>., ,, , 



pleopods ; IV- vii, th via, free thoracic of these two or three pairs have 



segments. (From Lang's Comparative pyonnr q: 1 - pe , ( pnr \ 

 Anatomy, after Sars.) eXOpOOlteS (6X). 



