88 M. L. Plate on Ectoparasitic Rotatoria 



the adult animals often remain seated for a long time in the 

 same place, or at least that they limit their locomotion within 

 very small distances. 



The most essential distinction which exists between the 

 genera Seison and Paraseison on the one hand, and the nume- 

 rous genera which I have classed together as " Ductifera " in 

 a recently published memoir * on the other, is this, that only 

 in the latter does a marked sexual dimorphism occur, while it 

 is wanting in the former. In the Seisonid^ males and 

 females are approximately of the same size, and the difference 

 of the two sexes finds expression only in the sexual apparatus 

 and, in connexion therewith, also slightly in the secretory 

 organ and muscular system ; all the other organs are of the 

 same nature in both. 



A glance at PL VI. fig. 1 shows us the external habit 

 of the animal. The body is divided into four distinctly 

 marked sections, which, as in the genus Set'son^ may be cha- 

 racterized as head, neck, middle-body or trunk, and abdomen 

 or tail. The neck and tail have a cylindrical form, while in 

 the two other sections the sides of the body are considerably 

 broader than the dorsal and ventral surfaces. The females 

 in the adult and fully extended state are of an average length 

 of 0*9-l'l millim. ; the males are rather smaller, attaining 

 only 0*4-0'7 millim. As, however, both the neck and the tail 

 are composed of apparent segments which can be invaginated 

 one within the other, and, further, tlie whole neck is fre- 

 quently retracted within the trunk, the Paraseison may 

 acquire a mucli smaller length of body. When an adult 

 individual is completely extended the abdomen is the largest 

 section of the body ; the trunk and neck are rather shorter, 

 but nearly equal to one another. The foremost section is 

 two thirds of the length of these. For systematic purposes 

 such relative sizes may be of importance, and therefore the 

 measurements of an old female example are here given in 

 millim. : — 



Total length M; tail 0*34: trunk 0-27; neck 0-28; 



head 0-19. 

 Width of neck at anterior end 0*0 16. 



„ ,, posterior end 0"04. 



Greatest breadth of trunk seen from the side 0*104. 

 Breadth of tail at the base 0'06. 



„ „ posterior extremity 0*02. 



* L. Plate, " Untersucliung einiger an den Kiemenblattern des Gam- 

 marus jmlex lebendeu Ectoparasiteii," m Zeitschr. f. wiss. Zool. Jid. xliii. 

 p. 229 (1886). 



