NO. 1404. 



/'. ! RASITH ' < 'OPEPOnS—CALIGIDJE— WILSON. 



495 



m. 



Rarely a second transverse groove is found in front of the crossbar of 

 the II as in CaUgus hsemulonis and C. alimicus and in Gloiopotes 

 ornatus (See Plates IX, XVIII, XIX, XXIII). 



The frontal plates (F. P.) are separated from the carapace in all the 

 genera here considered by an irregular groove made up of several 

 symmetrically arranged curves. 



These frontal plates are really the basal joints of the first antennae, 

 as can be readily seen in following the development, and they thus 

 correspond in function to the hooked claws on the base of the first 

 antemuv of the Argulidse. 



The two plates do not quite meet at the center, but are separated 

 anteriorly by a deep and very narrow sinus which marks the former 

 position of the filament for attachment in the chalimus stage, and by 

 a slight projection of the carapace from which this filament emerged. 

 On the ventral surface at the base of 

 the suture there is an oval opening 

 surrounded by a narrow fringe of 

 chitin. This represents the median 

 sucker which is considerably devel- 

 oped and forms an important organ 

 of adhesion in the early chalimus 

 stages (s. fig. 3). Its usefulness is 

 almost entirely superseded in the 

 adult ('(///)/ us by the sucking disks 

 which develop during the later chal- 

 imus stages, but in the other genera 

 it may serve as a "" first aid in secur- 

 ing the animal to its host," as sug- 

 gested by A. Scott (1901). 



In favorable specimens a chitinous 

 rod (f. f.) can be seen passing back from this sucker toward the eyes. 

 This rod is the remains of the filament, and at its inner end may be 

 seen the gland which secreted the substance of the filament. 



The sucking disks, which in Caligus serve both for prehension and 

 locomotion, are of peculiar construction, resembling not the body or 

 basal portion of the sucking disks in Argulus, but rather the free 

 membranous border. They consist of a short and rather flat cone of 

 membrane, often split for a considerable distance down the ventral or 

 anterior side. 



This suture is often so wide that a cross section would take the form 

 of a horseshoe rather than a circle. The membrane is supported by a 

 very few transverse and by many longitudinal ribs of chitin, all of 

 which, however, are simple hairs or threads and not the complicated 

 affairs found in the Argulidse. 



Fig. 3.— The median sucker of Caligus 



EAPAX. f. f., FRONTAL FILAMENT; m., AN- 

 TERIOR MARGIN OF FRONTAL PLATES; S., 

 SUCKER. 



