INTESTINAL WOKMS. 171 



Danish zoologist MUELLER classed amongst his Infusories minute 

 worms with tails, to which he gave the generic name of Cercaria. 

 The accurate observations of NITZSCH taught us to define the genus 

 more completely, and supplied a lively picture of the form and 

 movements of a worm that to the naked eye seems like a moving 

 point 1 . He compared these animals to Distomes that have had a 

 tail-like appendage attached to them, and their motions to those of 

 a Vibrio: each of these structures, the body and the tail, had a 

 separate motion : when the body moved on by creeping and sucking 

 the tail was at rest; and, conversely, when the tail undulated rapidly 

 it forced along with it the body, which seemed now to have no 

 independent motion of its own. NITZSCH further saw that Cercarice 

 cast their tail, and observed in Cercaria ephemera that it fixes 

 itself, covers itself with a shell that presently hardens, and thus 

 becomes, as it were, a pupa. Such pupae remain unchanged for 

 months: what was to succeed remained unknown. Afterwards 

 similar observations were made by others, particularly by VON 

 SIEBOLD. STEENSTRUP* shewed that the pupae change into Di- 

 stomes after the lapse of several months. Cercarice live as parasites 

 within the bodies of different Molluscs, as Lymnceus and Planorbis. 

 But this does not terminate the surprising series of these changes. 

 Cercarice, the larvae of Distomes, do not arise immediately from the 

 eggs of these last. Within the above-named Molluscs, and in some 

 others, little bags of an oblong form (germ-pouches) are found, in 

 which a peculiar organisation and motion may sometimes be traced, 

 but which in other species are motionless, and contain Cercarice in 

 a more or less advanced state of development. Considered as para- 

 sites of the germ-pouches, these have been named parasites of the 

 second order: yet they are not parasites, but the progeny of these 

 vermiform germ-pouches 3 . How these last originate from the young 

 of Distomes has not been absolutely ascertained. The young animals, 



1 C. L. NITZSCH Beitrag zur Infusorienkunde, Halle, 1817, 8vo. 



a STEENSTEUP Alternation of Generation. Translated by BUSK for Ray Soc. 



3 BOJANUS, who discovered such vermiform germ-sacs of a yellow colour in Lim- 

 nceus stagnalis, named them King's yellow worms; Isis, 1818, s. 729. The celebrated 

 V. BAER published many similar observations in Nov. Act. Cces. L. C. Nat. Cur. 

 Vol. xin. P. i, pp. 605 659, Taf. xxxi. ; as also the far-famed investigator of the 

 lower animal forms V. SIEBOLD in BUBDACH'S Pkysiol. 2 te Ausgabe, s. 186, &c. 



