135 323 



probably to press out the wheel-organ and foot by means of altering the pressure 

 in the body cavity, when the organs have been drawn in by means of tlie longitu- 

 dinal muscles. 



The longitudinal muscles maj' commonly be divided into two groups, those of 

 the anterior and those of the posterior part of the body; the former draw in the 

 wheel-organ, the latter the foot; where the foot is rudimentary or absent, they are 

 commonlj' greatly reduced. The two muscle groups overlap each other in the 

 middle part of the body. The number, place and function of these longitudinal 

 muscles are remarkably constant in the different genera and species, f. i. in the Flos- 

 culariidœ and Melicertidœ, where long longitudinal muscles pass through the whole 

 body from the wheel-organ to the end of the foot. They have only been more 

 thoroughlj' studied in very few species. The old division of Zelinka in "Haut und 

 Leibeshöhlemusculature" has now been discarded. It seems as if w-e find striated as 

 well as smooth muscles; in some species special muscles, especially those of the 

 wheel-organ, are conspicuously striated; Martini (1912, p. 614) maintains that, in 

 Hijdatina, excepting only two muscles, he only finds striated ones. Whether the 

 muscle elements are to be regarded as muscle threads or muscle cells, cannot be 

 stated with certainty. With regard to Hydatina, Martini has arrived at the main 

 result that all the transversal muscles and all the more significant muscles are really 

 muscle cells, muscle individuals with one nucleus onlj' (exceptionally two). 



Muscles. Males. The nature of the investigation prevents any elucidation 

 of the histological structure of the muscles in the male sex; the division of the 

 muscles in transversal muscles, longitudinal muscles, and those belonging to special 

 organs, may also be maintained for the male sex; in most of the males we find 

 from five to six transversal baiîds, most conspicuous in the more primitive families, 

 in a few of them f. i. Hijdatina even more. In some of the males, belonging to lo- 

 ricate families, the transversal bands are even more conspicuous in the males than 

 in the females, (Brachionidœ) but, especially in the males of the plancton Rotifers, 

 not the slightest trace of a transversal muscle system has been detected (Pedalion, 

 Triartbra, Polyarthra, Poinpholyx a. o.). The longitudinal muscles are highly deve- 

 loped, especiallj' the anterior group, in all the more primitive families, and where 

 the foot is absent in the males, the posterior group is very much reduced. 



Muscle's belonging to special organs are almost absent; almost only the muscles 

 of the penis belong to these and of the contractile vesicle of the Asplanchna ; but in 

 the most reduced males this seems almost to be lacking, the penis being protruded 

 by means of alternation in pressure in the body cavity. 



Body cavity. The body cavitj' of the Rotifera is filled with a fluid the na- 

 ture of wliich is entirely unknown to us. In some Rotifers, f. i. the Asplanchna, it 

 is extremely large, all the organs lying in their whole extension bathed in the fluid ; in 

 others it seems that the thickness of the hypodermis diminishes its space in a very 

 high degree. As the hypodermis is thickest in the young, just born individuals, it 

 will be understood that its size is augmented during growth. In the fluid are found 



