144 THE CRUSTACEA 



male apertures on the eighth trunk-somite ; paired eyes usually 

 present ; development usually with metamorphosis, young rarely 

 hatched in nauplius stage. 



The sub-class Malacostraca includes such a diversity of forms 

 that it will be necessary to deal more fully than in the case of the 

 other sub-classes with the separate orders composing it. Before 

 doing so a brief account must be given of the general type of 

 organisation found throughout the sub-class. 



Apart from the fixed number of somites, to which the Leptostraca 

 offer the only exception, the most characteristic feature of the 

 Malacostraca is the separation of the trunk -limbs into sharply 

 defined thoracic and abdominal tagmata. This, together with the 

 constancy in position of the genital apertures, on different somites 

 in the two sexes, is sufficient to demonstrate the unity of the 

 sub-class. 



Leaving the Leptostraca aside for the present, the more primi- 

 tive members of each of- the " divisions " in the scheme of classi- 

 fication here adopted approximate to a common type of structure 

 from which the more specialised members of each group diverge 

 very widely. Thus, the possession of a carapace enveloping the 

 thoracic region, movably stalked eyes, biramous antennules, a 

 scale-like exopodite on the antenna, natatory exopodites on the 

 thoracic limbs, an elongated and ventrally flexed abdomen, and a 

 " tail-fan " formed by the lamellar rami of the last pair of append- 

 ages spread out on either side of the telson, are characters common 

 to the Mysidacea, Euphausiacea, and the lower Decapoda, and, 

 with some modifications, to the Anaspidacea and Stomatopoda. - It 

 seems reasonable to suppose that this combination of characters, 

 making up what has been called the "caridoid facies," must be 

 attributed to the hypothetical common stock of the Malacostraca, 

 At all events, it is possible to represent, in diagrammatic fashion, 

 a generalised Malacostracan which serves as a convenient summary 

 of the morphology of the group (Fig. 85). Some of the characters 

 of this type require to be considered in more detail. 



The antennule, as already stated, is biramous, having two 

 flagella springing from a peduncle of three segments. Since the 

 antennules in the other sub-classes are always uniramous, as they 

 are in the nauplius, it seems probable that the two flagella do not 

 represent the endopodite and exopodite. When only one flagellum 

 is present in the Malacostraca it is the outer which persists, and 

 it alone, as is shown by the sensory filaments Avhich it carries, 

 corresponds to the single ramus of the other sub-classes. In certain 

 Decapoda (Caridea) and in Stomatopoda there are three flagella, 

 the outer flagellum being divided into two. 



The protopodite of the antenna is composed of two or of three 



