326 138 



commonly on the last segment but one: the opening is almost always marked by 

 means of a tuft of cilia. In the pairing moment the vas deferens in these forms is 

 evaginated turned inside out, now showing a hyaline tube or cupshaped body, 

 commonly at its base surrounded by a wreath of cilia, the same which originally 

 presented itself as a tuft round the opening; a penis of this kind is found in all 

 the Xofommaticlœ hitherto observed, in Hydatina, in the Flosculariicke and in the 

 Melicertidæ, where it is placed on the very apex of the animal; it is of quite the 

 same type in Asplanchna. 



In many Rotifers we find a penis of a somewhat different kind. Round the open- 

 ing for the penis on the dorsal side of the foot, the skin is folded, often somewhat 

 resembling the so-called tail in the genus Copeus. By means of the transversal 

 muscles and during the bending of the body, the fluid of the body cavitj' is pressed 

 out into this part of the body; this is now altered into a stiff erectile organ, often 

 much longer than the foot which only hangs down as a little ventral appendage. 



A penis of this kind is most conspicuous in the Brachionidce, most probably 

 also in Enchlanida' and Salpinadæ and in Gastropiis hyptopus. In these species the 

 vas deferens is much longer than in the former group, and capable of great alterations 

 with regard to length; as in Hydatina it is often transversally banded, most probably 

 by very fine muscle threads. 



In the Anurœadœ where both sexes lack a foot, the penis is always in an erect 

 condition, hanging down from the posterior end; it cannot be drawn in. A penis of 

 this peculiar t}'pe is best developed in Aniiræa hypelasma. 



In the most reduced males we find a penis of quite another type; here is 

 really no penis at all. In the pairing moment the whole posterior part of the body 

 is prolonged and often formed like a tube; this is most conspicuous in Polyarthra, 

 Pedalion and Triarihra, where the posterior part of the body, when the animal is 

 swimming, is rounded, but when used as penis, it is tapering. As mentioned above 

 we commonly find a tuft of hairs at the opening, but in some species, f. i. of the 

 genus Ånuræa, we find two lateral hairs ; this is also the case with Triarthra but 

 here the opening is further supported by a chitinous frame. 



The length of the vas deferens differs very much, and is in accordance with 

 the structure of the penis; it is almost always coated with cilia; the walls are thin, 

 transversally striped with muscle threads. In some cases f. i. Brachionus and Diglena 

 the last part of vas deferens is transformed into a chitinous tube which, when used, 

 is forced out of the genital opening. In Brachionus it ends in a cupshaped dilation; 

 this is also the case with the penis in the genus Raftulns. In Copeus labiaius it is 

 widened to a large sac. 



At the sides of the vas deferens are two, rarely four glands, commonlj' design- 

 ated as prostata glands. 



They may be very large as in Asplanchnopus and Hydatina and where the}- 

 are not indicated, it is most probably because they have been overlooked. In some 

 species, f. i. Polyarthra and especially G. hyptopus, they have a peculiar lobate ap- 



