THE DECAPODA 



265 



end of the peduncle. The proximal segment of the peduncle, which 

 in most cases lodges the statocyst, possesses in the Natantia a very 

 characteristic expansion of its outer margin in the form of a rounded 

 lobe or spiniform process known as the stylocerite (sty). In the 

 Brachyura the flagella are very short or quite vestigial ; the basal 

 segment is enlarged and generally firmly fixed in the antennular 

 fossa, and the other two segments fold up beside it. 



Certain special modifications of the antennules may be men- 

 tioned here. In the Sergestidae the outer flagellum of the male 

 is bifurcated and forms apparently a prehensile organ. In Hymeno- 

 cera (Caridea) the inner flagellum is broadly foliaceous. In 

 Solenocera (Penaeidae) the same flagellum is in the form of a 



B 



--t. 



Fio. 150. 



A, antennule, B, antenna, of Athanas nitescens. (After Sars.) 1-5, segments of the peduncle 

 (the fourth segment in the peduncle of the antenna is not visible from above) ; ex, "scale" 

 or exopodite of antenna ; i, inner flagellum ; o, outer flagellum of antennule with its inner 

 branch bearing olfactory filments ; st, statocyst in basal segment of antennule ; sty, stylo- 

 cerite ; t, tubercle bearing aperture of antennal gland. 



half-tube ensheathing the outer flagellum and forming with its 

 fellow of the other side a long siphon supposed to have a respira- 

 tory function. In Albunea (Hippidea), where by a rare exception 

 only one flagellum is present, a respiratory siphon is formed by the 

 apposition of the two antennules, which bear each a double longi- 

 tudinal row of setae. 



In the lower Decapoda the peduncle of the antenna has five 

 segments, the two segments of the protopodite and the first three 

 of the endopodite, but the segments are usually more or less 

 displaced so as to articulate with each other in a zigzag manner 

 (Fig. 156, B). The exopodite (ex) forms a large foliaceous "scale" 

 (squama) in the Natantia. In most Reptantia the number of 

 peduncular segments is reduced by the fusion of the second and 



