GENUS TELOTROCHIDIUM. 643 



rapidly backward and forwards upon it as if on a pivot. It is under the last 

 named circumstances that the flexibility of this organ is manifest. When closely 

 observed it is then seen that with each rotation it is thrown into a twist of many 

 coils after the manner of a pliant band or cord, the next rotation relaxing this 

 flexure and coiling it in a similar manner in an opposite direction. The aspect of 

 the caudal appendage in its twisted state is so like that of a bundle of smaller 

 thread-like filaments or of a long drawn-out and sharply pointed screw, as observed 

 by Professor Clark of the homologous organ in Calceolus, that it is difficult to refrain 

 from a suspicion that in that type also it may eventually prove to possess a similar 

 band-like character. It is at the same time requisite to remark that in some 

 instances the caudal appendage of that organism appeared to him to be divided into 

 equal halves, representing possibly, however, an abnormal development, while at 

 other times it presented distally a brush-like aspect. 



The contractile vesicle in Urocentrum is a very conspicuous object, exhibiting 

 at full diastole, as seen in profile, apparently two, but actually four, rounded lateral 

 sinuses whose diameter equals about one-half of that of the large spheroidal central 

 lacuna. With the act of systole the greater portion of the liquid contents is discharged 

 into the outer water, leaving, however, a small portion behind in each of the four 

 sinuses ; these now become distended, and coalescing with one another form the 

 characteristic spheroidal central vesicle, to which again lateral diverticula are added 

 and systole is repeated as before. The various aspects of the contractile vesicle 

 during these successive stages is shown at PI. XXXIII. Fig. 10, a to f, the 

 phenomena as a whole corresponding closely with what has been recorded by 

 Wrzesniowski of Enchelydon farctus and Trachelophyllum apiculatum. In August 

 1872, the author met with a salt-water representative of the genus Urocentrum 

 at Bognor, Sussex. From the sketches and brief notes made at the time, it would 

 appear to present little or no points of distinction from U. turbo ; the triangulation 

 of the posterior region of the body was perhaps more decidedly marked, while the 

 caudal appendage appears to have been more curved and claw-like. It is proposed 

 to provisionally distinguish this salt-water form of the species under the name of 

 Urocentrum turbo var. marinum. Under the action of dilute osmic acid the 

 animalcules of U. turbo speedily disintegrate, demonstrating the absence of an 

 indurated cuticle or cuirass, which under normal conditions might be suspected 

 to exist 



GENUS III. TELOTROCHIDIUM, S. K. 



(Telotrocha, worm-larva ; eidos, form.) 



Animalcules entirely free-swimming, ovate or campanulate, possessing 

 no caudal appendage ; ciliary girdles two in number ; oral aperture opening 

 on the ventral surface, immediately behind the anterior wreath of cilia; 

 anal aperture postero-terminal ; contractile vesicle and endoplast con- 

 spicuously developed. Increasing by longitudinal fission. Inhabiting 

 fresh water. 



Telotrochidium crateriforme, Mull. sp. PL. XXXIII. FIGS. 11-13. 



Body campanulate or subquadrate, with an indented dorsal and 

 convex ventral or oral aspect ; ciliary wreaths developed at a short distance 

 only from the anterior and posterior extremities, the anterior one associated 

 with a thick annular border ; anal aperture postero-terminal, tubular, 

 permanently visible ; contractile vesicle single or jdouble, subcentral ; endo- 

 plast band-like, curved ; parenchyma transparent, pale brown or amber- 



N 2 



