568 



ZOOLOGY 



SECT. 



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

 the telson and the expanded uropods. 



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 : Diastylis (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 

 absent. The carapace (cth) is so 

 small as to leave the five posterior 

 segments (th IV VIII) uncovered. 

 The first two pairs of thoracic limbs 

 are maxillipedes, the last six, legs : 

 of these two or three pairs have 

 exopodites (ex). 



\ 



FIG. 467. Diastylis stygia. 01, an- 



tennule ; 0% , antenna ; 06.1 ab.-j , ab- 

 dominal segments ; cth. cephalothorax ; 

 en, endopodite ; ex, exopodite ; p.i, p.$ , 

 pleopods ; IV-VII, th VIII, free thoracic 

 segments. (From Lang's Comparative 

 Anatomy, after Sars.) 



