MORPHOLOGY OF CYCLOPS. 7 



The first sternite is the longest from hefore backwards. The fourth has no pentagonal 

 plate, but only a ring behind the insertion of the limb, and sends up a pair of short lateral 

 apodemes for the partial insertion of part of the great pectoral flexors of the body. 



In the fifth thoracic segment the sternal thickenings are simply a succession of rings 

 incomplete dorsally, of which the last (possibly belonging to the sixth segment) sends in a 

 strong median process for the insertion of the chief pectoral flexors of the body. 



The five succeeding segments narrow gradually ; each is nearly cylindrical and over- 

 laps its successor. The last thoracic segment is much swollen ventrally in both sexes, 

 containing the vesiculce seminales or the anterior part of the spermatheca, as the case 

 may be ; in the female it is united with the first true abdominal segment. 



The proper abdominal segments have their posterior overlapping edge fringed with 



teeth, sharp and elongated in the last at the base of the furca. I give the formula of 



the relative lengths of the segments, furca (f), and caudal setae (s) — 



th. vi. abfl. i. ii. iii. iv. f. s. 



male. 



11 10 5 4 4 14 52 



th. vi + abrl. i. ii. iii. iv. /. s. 



24 9 8 7 20 66 



female. 



The unit of these measurements =-^g mm - 



The upper surface of the posterior half of the last abdominal segment is excavated, 

 with the adjoining parts of the furca, into a squarish supra-anal cavity, into which the 

 anus opens as a longitudinal cleft. Over this the tergum is prolonged as a semicircular 

 plate, called by Claus " Afterklappe," but which is immovable and should rather be termed 

 "Afterdecke." Elsewhere * I have called this the supra-anal plate (PI. III. fig. 2), and shown 

 that it is equivalent to that part of the telson of the Decapoda which projects beyond the 

 anus, while the furcal lobes exist, even in the Astacinse, as Little, oblong, setose, retral pro- 

 cesses. The furcal processes I regard as equivalent, if not to limbs, to the paired outgrowths 

 of the body that develop elsewhere into limbsf ; if they have no muscles, it is because their 

 position on a terminal segment makes them unnecessary. I should not be surprised to find 

 them movable, with proper muscles, in some member of this group. A small crown of 

 teeth surrounds the distal end of the furcal process. The process is setose on its inner side, 

 and bears on its dorsal surface a flexible plume, about one third its length from its distal 

 end, and a similar one at its outer side. At the end are four long pointed spines with (hori- 

 zontal) pinnately attached setse. Of these spines the outermost is the shortest, the inner 

 twice as long (longer than the furca), the second outermost six times as long, and the 

 second innermost seven and a half times as long, nearly as long as the after-body and 

 furca together. Both of these long plumes are articulated by an internal thinning of 

 their cuticle a little above their base. 



This is a convenient place for a list of measurements of two adult specimens, male 

 and female %. 



* British Association Report, 1882. 



t In development they agree with limbs. They do not exist in the youngest Nauplius, but after the first moult 

 appear as distinctly ventral outgrowths. 



J The specimens from which these measurements were taken were confined with as little pressure as possible to 

 avoid distortion, and I do not think that the width is much exaggerated, if at all. 



