PARASITIC COPKPODs— WILSON. 



841 



distal ends. There are three large eyes in a transverse row just in 

 front of the center of the carapace, the central one smaller than 

 the other two and slightly posterior to them. The frontal gland and 

 attachment filament secreted hy it are very different from those found 

 in the Caliginie. 



The gland is a huge quadrangular affair, filling the entire center of 

 the front of the carapace. In place of the two large gland cells seen 

 in the Caligina\, we find liere four, " arranged in two pairs, one on 

 either side of the mid line. 



Each cell is ellipsoidal in form, the ones nearest tli(^ mid luie l)eing 

 larger and farther l^ack than tlie outer ones. 



In place of the single slen- 

 der and cylmdrical filament 

 produced m the Caligin^e, we 

 fhid here in PerissojMs two 

 broad and ribbon-lil^e fda- 

 ments placed side by side. 

 Each is strongly compressed 

 dorso - ventrally, and is 

 barely long enough to reach 

 the fish's skin and obtain a 

 firm hold. The chalimus is 

 thus held with the frontal 

 margin of the carapace 

 almost in actual contact 

 with the fish's skm, and its 

 condition is very different 

 from that of the Caligus 

 chalimus which floats out at 

 the ti]) of a filament as long 

 as its own body. 



When these double fila- 

 ments disappear in the adult 

 they leave a broad and well- 

 defined sinus at the center of the frontal margin, between the frontal 

 plates. In all the adult Nogaus males this central sinus and the 

 remains of the frontal gland can be clearly seen, often with shreds 

 of the frontal filaments still attached at the base of the sinus, but 

 in the adult females the glands and sometimes the sinus disappear. 

 In younger females, however, they can still be distinguished. 



In the size, arrangement, and general appearance of the free thorax 

 segments of this chalimus there is a striking resemblance to the male 

 form (see p. 357). The latter sex, therefore, must be taken as the 

 typical form in this subfamily, and from this the female degenerates 

 on becoming fixed. 



Fig. 12.— The foue pairs of swimming legs of the 

 rhalimus shown in tiik preceding figure. 



