Parasitic Copepod Tliersitina gasterostei. 419 



appendages — mandibles, first maxillae, and second maxilla? 

 (PI. XI. fig. c), — but in the male a pair of maxillipedes are 

 also present (PI. XI. fig. a). The mandibles are strong 

 wedge-shaped organs with a toothed cutting-plate and an 

 accessory toothed lobe running upwards into the mouth. 

 This lobe is overhung by the free edge of the labium, which 

 is a delicate plate largely fused with the cuticle of the head. 



The first maxillae are very rudimentary, and consist of a 

 pair of minute knobs bearing three short spines (PI. XI. 



The second maxilla? consist of a large basal part, divisible 

 into two joints, and a distal curved joint with a toothed end 

 (PI. XL. fig. e). This distal joint is very freely movable, 

 and can generally be seen in active movement when the 

 living animal is examined. 



The maxillipedes appear first in the last larval stage of the 

 male as a pair of simple knobs, and are fully developed at 

 the next moult. In the adult male the maxillipede is three- 

 jointed, the first joint fused with the head, but the other two 

 freely movable. The second joint is broad and armed with a 

 row of small spines, and the distal joint is a long curved 

 clasping rod, sometimes showing an apparent division at the 

 base into two joints. 



According to Dr. Scott, the mouth-parts consist of man- 

 dibles, maxilla?, and two pairs of " maxillipedes" ; and in his 

 more recent account* he appears to confirm his original 

 description. 



I have examined a number of specimens, and have in no 

 case found more than three pairs of appendages in the female, 

 so that it is clear that these appendages do not differ in 

 number or position from those of Ergasilus. Wilson (1911, 

 p. 283) has given reasons, with which I fully agree, why the 

 two pairs succeeding the mandibles should be designated as 

 maxilla? and the pair following them in the male as maxilli- 

 pedes or thoracic appendages. A full discussion of the 

 mouth-parts of Tliersitina and Ergasilus will be found in 

 "Wilson's paper. 



The swimming-legs. — The first three pairs of legs have both 

 rami three-jointed and of approximately the same length. 

 The two basal joints are very broad and the legs of each pair 

 are united by a strong chitinous bar attached at its ends to 

 the first basal joint. The second joint bears a seta on its 

 outer face and two rows of small spines on its inner edge. 



* 'British Parasitic Copepoda' (Ray Society, 1913), p. 42. 



