250 DE. G. H. FOWLEE — BISCATAN PLANKTON : 



value would be seriously weakened if so great a range of variation were possible as is 

 sbown in Miiller's figures, pi. xvii. 23, 24, and 25, 26. Fortunately, it appears tbat tbis 

 extreme range of possible contour is due, not to individual variation, but to tbe different 

 shapes of tbe sbells at Stages I. and II. So far as I bave seen (witb tbe exception of 

 one abnormal specimen*) all specimens at Stage I. were of tbe elongate type; at 

 Stage II. tbey are never so elongate as at Stage I., altbougb both exhibit a certain 

 amount of individual variation. At tbe earlier stage tbey become even more rounded. 

 A glance at tbe figures of tbe shells on Plate 24 will show the changes of form which 

 accompany the successive metamorphoses. 



For reasons which will appear in the Developmental Part (p. 273), the means are only 

 wide approximations. 



Stage I. 



Male (Mean, 1-0). — Shell very elongate ; tbe height to the length as 1 to 1-9 ; tbe 

 opening of the left asymmetrical gland far forward, near tbe rostral incision. The 

 shoulder-ridge slight ; the posterior angle markedly rounded, the posterior border 

 strongly convex, the ventral angle markedly rounded ; tbe ventral border nearly straight 

 and generally almost parallel to tbe dorsal border, tbe shell being very slightly highest 

 in the posterior third. Frontal organ and Antenna i. as described and figured by 

 Miiller (xvii. 29, 34) ; the armature of the principal bristle consists of about 10-12 pairs 

 of spines, of which the distalmost five or six pairs are close together, the more proximal 

 are placed at gradually increasing intervals. Antenna ii. as described by Miiller; 

 there are two very small basal bristles ; of the central bristles tbe one is about twice 

 the length of the other ; both are hairless. 



Female (Mean, I'l). — Shell very elongate; tbe length may be more than twice the 

 height ; the shell is sometimes more markedly highest in the posterior third than is 

 sbown in fig. 215. Tbe posterior border is more sharply convex than in the male, 

 but otherwise similar to it. Frontal organ rather variable: the normal type as in 

 fig. 216 (compare Miiller's figures, xvii. 30 to 33). Antenna ii. witb two hairless 

 central bristles, tbe longer about twice tbe length of tbe shorter ; sometimes with two 

 tiny spines in the position of basal bristles. 



Stage II. 



Male (Mean, 0-75). — Shell much less elongate. The height to the length about as 

 1 to 1-7. From tbe posterior end of the dorsal border runs a steady sharply-convex 

 curve, which is unbroken till just below the rostrum, obliterating tbe posterior angles. 

 The greatest height is near tbe middle of the length, the ventral border strongly convex. 

 Frontal organ with short, sharply segmented, terminal joint; tbe dorsal border 

 depressed about tbe middle of its length, with a few fine spines proximally; tbe ventral 

 border correspondingly swollen, then constricted, with numerous fine spines, ending 



* The specimen, a male of 1*0 mm., that is to say of the length of Stage I., had not developed secondary sexual 

 characters — the armature of the principal bristle of antenna i., the hook-joint on antenna ii., and the form and 

 spinulation of the frontal organ ; it had been arrested at Stage III. Such cases of arrested sexuality have been 

 described from other groups ; the testes were rather small, but I could see no trace of " parasitic castration." 



