AETHROPODA. 29 



modified appendages, to the number of twenty pairs. These are 

 named in order from the first segment as follows. In front of 

 the mouth are a pair of eyes, a pair of antennules, and a pair 

 of antennae. Behind the mouth are a pair of mandibles, two 

 pairs of maxillae, three pairs of maxillipeds, five pairs of ambu- 

 latory legs, and six pairs of swimmerets: while to the last 

 sub-abdominal segment is attached a median azygos element, 

 the 'telson.' 



A sub- abdominal 'somite,' which may be regarded as a 

 typical one, is made up as follows. It is nearly semicircular 

 in vertical section, the dorsal wall or 'tergum' being very con- 

 vex, and the ventral wall or 'sternum' nearly straight or 

 but slightly convex. At the infero-lateral angle both the ' tergum ? 

 and the 'sternum' are produced downwards and outwards into 

 a lobate hollow process, termed the 'pleuron.' In the angle 

 between the 'sternum' and the inner surface of the 'pleuron/ on 

 each side, is an articular cavity, which receives the proximal 

 extremity of the appendage attached to this 'somite.' A trans- 

 verse groove marks off a crescentic area on the anterior surface 

 of the 'tergum,' which is overlapped by the hinder part of the 

 'somite' in front of it. This area is the tergal facet. A similar 

 flattened surface exists on the anterior portion of each 'pleuron.' 

 This is the pleural facet. Each appendage consists of a basal 

 attached portion (divided longitudinally into two segments), the 

 'protopodite.' Each' pro topodite' (sometimes called 'basipodite') 

 carries at its distal extremity an external and internal fila- 

 ment, to the former of which the term 'endopodite,' and to the 

 latter 'exopodite/ has been applied. 



This description will in the main apply to the four central 

 sub-abdominal segments, but the first and the sixth of these 

 segments have their appendages somewhat modified for special 

 purposes. In the sixth sub- abdominal segment, the 'exopodite' 

 and 'endopodite' are enormously expanded into flat setose plates, 

 and the exopodite is further divided by a transverse joint. 



The 'telson' is a flattened conical setose plate, attached 

 to the hinder margin of this * somite,' and forms, with the 

 expanded ' exopodite ' and ' endopodite,' a powerful swimming 

 tail. 



In the first sub-abdominal 'somite ' each appendage consists of 



