NO. 1788. 



NORTH AMERICAN ERGASILID^— WILSON. 



271 



carapace a large anterior shield connected with the bases of the second 

 antennae (fig. 2). This latter groove does not in any way indicate 

 the limits of the cephalon, since the mouth-parts are situated pos- 

 terior to it on the ventral surface. Furthermore, in most of the 

 species no groove at all is visible, and consequently it is not feasible 

 to divide the carapace into areas as was done in the Argulidae and 

 Caligidse. The cuticles covering the dorsal and ventral surfaces of 

 the cephalothorax are fused along the margins, but they do not form 

 a flexible border as in the Caligidae. In those species of the Bomo- 

 lochinse and T8eniacanthina3 in which the carapace forms a sucking 

 disk for prehension the edges of this disk are formed of the ventral 

 cuticle rather than of a fusion of the two cuticles. 



The eyes are situated close to the anterior margin of the carapace 

 and are fused on the mid-line near the ventral surface. Their inner 

 margins, in contact with each other, are 

 heavily pigmented, while the outer portions 

 are clear and transparent. No definite lens 

 is visible. In most of the species of the 

 Bomolochinse and Tseniacanthinse the eyes 

 are invisible, but this is probably due to the 

 inflated condition of the cephalothorax and 

 the opacity of its contents. They are cer- 

 tainly present in the genus Ergasilus and in 

 some species of Bomoloclius {teres), Artacolax 

 (cornutus), and Thersitina (hiuncinata). 

 They do not change their position during 

 development as in the Caligidse, but appear 

 in the same place in the youngest nauplius 

 stage as in the adult. 



Free thorax and genital segment. — The tho- 

 rax is composed of six segments, the first of 

 which is usually joined with the head, while 

 the sixth constitutes the genital segment, thus leaving four free seg- 

 ments, each of which bears a pair of swimming legs. In such males and 

 immature females as have been found the genital segment also bears 

 a pair of rudimentary legs, but these can not be seen in most of the 

 adult females (see pi. 58, fig. 200). The thorax segments usually 

 diminish regularly in size from in front backwards ; the fifth is rudi- 

 mentary and very short, and is often so overhung by the preceding 

 segments as to be invisible in dorsal view. The legs which it bears 

 are also rudimentary, uniramose, and one or two jointed. They are 

 sometimes reduced to mere papillae, each bearing one or more short 

 spines, or even to simple spines without any papillce. In the Caligidss 

 evidence was found that the so-called genital segment is really a fusion 

 of two segments, the anterior of which, the fifth segment, is the larger. 



Fig. 2.— Dorsal sukface of cara- 

 pace IN Ergasilus centrarchi- 



DARUM, SHOWING THE LARGE AN- 

 TERIOR SHIELD CONNECTED "WITH 

 THE BASES OF THE SECOND AN- 

 TENNA. Fig. 11 (p. 286) SHOWS 



THE POWERFUL MUSCLES AT- 

 TACHED TO THIS SHIELD. 



