6 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



the siphonal tentacle is a greatly enlarged and specialized marginal 

 tentacle.* 



Both structure and experiment agree in assigning to these tentacles 

 a tactile function. The siphonal tentacle is not, as might be supposed, 

 the most sensitive to touch of any of the organs. It seems to be 

 rather more sensitive than the marginal tentacles, about as sensitive 

 as the expansions of the mantle opposite the extremities of the stripes on 

 the posterior end of the shell, and rather less sensitive than the projections 

 opposite the extremities of the stripes on the anterior end of the shell, the 

 foot, and the siphons. It may be moved about with the point of a pencil 

 without causing much disturbance, but if it be pressed slightly, moved 

 quickly, or jarred, it is quickly withdrawn, and the withdrawal may be 

 accompanied by the partial or complete retraction of siphons, the closing 

 of the shell, and, in some cases, by the activity of the foot. It seems 

 especially sensitive to sudden movements or jars, and its special func- 

 tion may lie in this direction, in enabling the creature to detect the 

 approach of enemies, of which flounders seem to be among the most 

 dangerous. 



If a vessel, containing several specimens partially buried in the 

 mud, is not disturbed for some hours and is then jarred, all will 

 generally disappear with astonishing rapidity. All of the experiments 

 that were tried to determine the function of the tentacle resulted in 

 failure, inasmuch as specimens in which the tentacle had been removed 

 seemed to be as sensitive to jars as uninjured specimens. It may 

 be remarked, however, that the jars of a small vessel can at best 

 bear only a slight resemblance to jars affecting the surface of the 

 mud on the bottom of the ocean, such as would be caused, for instance, 

 by the swimming of a flounder. 



Posterior expansions of the mantle. These expansions, Figs. 1, 3 and 

 10, pe., lie opposite the extremities of the stripes on the posterior end of 

 the shell. They consist of flat, somewhat triangular extensions of the 

 edges of the mantle. The anterior border of each is fringed with 

 tentacles, resembling the ordinary marginal tentacles, but more 

 frequently branched, and usually with a rather larger proportion of 

 sense-papilla?. The inner surfaces of the expansions are very densely 

 ciliated. Near the base of each is a fold or ridge, along the side of 

 which such dirt as gains access to the mantle chamber is swept. 



* See note at the end of the paper. 



