41 8 CROWTHER : BIOLOGY OF SPH^RIUM CORNEUM. 



their incurrent siphons are inferior in position and in suspension 

 superior. After remaining in suspended rest for some time, 

 the animals lower themselves in jerks, as if a certain amount of 

 thread-slime were run out, and then comes a stoppage, until the 

 shells cease to oscillate from the jerky action of the fall, and 

 again more thread is run out and the process repeated until the 

 animals are satisfied with their quarters or the anchoring points 

 give way, and the molluscs fall to the bottom, soon to seek the 

 higher levels. Specimens anchored to the sides of the vessel 

 and which as a rule are tied by web-like arrangements of threads 

 often remain so for days, and when they crawl away leave the 

 sprawling filaments filled with effete particles. The method of 

 attachment here aids the molluscs in breaking free from useless 

 threads. Hence it is reasonable to suppose that mucous fila- 

 ments are secreted as needed. Sphcerium cannot crawl up 

 its thread. 



When undisturbed the suspended animal has the shell 

 gaping a little and the umbo downwards. From the posterior 

 end of the shell are extended, in each specimen, a pair of 

 united siphons ; from the anterior end a cylindrical foot. The 

 longer of the two siphons has its aperture reflexed and ciliated, 

 is usually turned upwards, bends gracefully and slowly about, 

 and organic particles in the water are in a constant rush to it. 

 This is the incurrent or afferent siphon. Somehow many small 

 infusorians — Vorticellce. and Faraincecia especially were watched 

 — when they come within the vortex of the incurrent stream 

 are dashed outside it just as they come within the play of 

 the siphonal cilia. The shorter of the two siphons, which is 

 united for two-thirds its length with the incurrent siphon, is the 

 excurrent or efferent siphon. Its orifice is contracted, ciliated, 

 and in the suspended animals is turned downwards. The 

 decrease in the orifices of the excurrent siphons gives greater 

 ejective force to the outflowing current and their tendency to 

 point downwards and away from the animals aids the gravitation 

 of the ejectamenta. A wonderfully simple but effective method 



J.C, vii., Oct. 1894. 



