GENERAL CONSIDERATION. 247 



The terga of tlie cephalic segments are thickened to form the 

 carapace (dorsal shield) ; this, at its lateral and posterior margins, 

 passes into an integinnental fold which runs back over the thoracic 

 region, and thus covers some or all of the thoracic segments dorsally. 

 Only rarely do the terga of the thoracic segments covered by the 

 dorsal shield retain any degree of independence (Stomatopoda, Fig. 

 141, p. 298, a few Schizopoda and Nehalia) ; in most cases the terga 

 of the thoracic segments fuse closely with the integumental fold of 

 the dorsal shield lying above them. A fusion of the cephalic and 

 thoracic segments thus takes place, forming a common region of 

 the body {cephalo-thorax). In one series of Malacostracan forms, 

 however, the marginal fold of the dorsal shield has undergone 

 degeneration (Arthrostraca) ; here, as a rule, only the anterior 

 thoracic segment fuses with the cephalic region forming the small 

 cephalo-thorax of this group, which is followed by seven free and 

 movable thoracic segments. 



The obliteration of the boundaries and consequent loss of move- 

 ment between the segments of the cephalic and thoracic regions just 

 mentioned has its effect on the condition of the limbs. Only in 

 rare cases {Nehalia, Euj)hausiidae) do all the eight pairs of thoracic 

 limbs agree more or less in structure. As a rule, one or more pairs 

 belonging to the anterior part of the thorax enter into close relation 

 with the mouth and become modified for the purpose of mastication, 

 'i'hese are tlien distinguished as maxillipedes, while the succeeding 

 thoracic limbs which serve for locomotion receive, in many groups, 

 the name of ambulatory limbs. In the Arthrostraca, only the 

 anterior pair of thoracic limbs is changed into a pair of maxillipedes, 

 but in the Decapoda there are three pairs of maxillipedes, and in 

 the Stomatopoila as many as five anterior pairs of thoracic limbs 

 are thus transformed. 



We must assume as the fundamental form of Malacostracan limb 

 a biramose swimming limb with basal epipodial appendage, such as 

 is retained as a thoracic limb in the Schizopoda. The shape of 

 the thoracic limb in Nehalia (Fig. 91 -0, p. 194) suggests to us that 

 this form may perhaps have been developed from a more lamellate 

 type of limb resembling that of the Phyllopoda. A two-jointed 

 protopodite passes into a five-jointed endopodite, while the exopodite 

 (flagellate branch), which often undergoes degeneration, frequently 

 exhibits a large nuuiber of closely-crowded joints beset with setae. 



If we compare the metamorphosis of most Entomostraca (especially 

 that of the Phyllopoda) with that of the Malacostraca (p. 193), 



