Mr. W. Clark on the Animal of Dentalium Tarentinum. 325 
only to consider that the water in this genus flows to the 
branchiz by the posterior aperture instead of at the front ; this 
view removes every difficulty, and may be regarded as a demon- 
stration of this fact, which is satisfactory and decisive, because 
it is founded on the organization which nature has conferred on 
these animals. 
I will state some facts in support of the conclusion that the 
branchie in Dentalium receive the water posteriorly. I admit 
that notwithstanding a constriction, it may possibly enter in 
front under the mantle and be discharged posteriorly, and vice 
versa ; but this action would be contrary to the natural position 
of the organs and the evidence I shall now adduce. But first it will 
be necessary to mention the mode of fixture of the animal to the 
shell: this is not at the centre, as in the spiral Gasteropoda, but 
at the posterior end, a little more than an eighth of an inch from 
the terminus, where, on the inner surface, may be seen the strie, 
in the hoilows of which the fine filaments issuing through the 
mantle and proceeding from the longitudinal elastic ribbon 
running from the foot are deposited, and together with the 
strong sphincter of the posterior process, which is imbedded in 
an indentation not visible from without, firmly secure, by con- 
striction, that end of the animal to the shell. This is a striking 
example of the admirable adaptations of nature of the organs of 
animals to their wants and ceconomy ; for if this animal was fixed 
to the middle of the shell as in the spiral ones, the contracti- 
bility of the posterior part of the body would be destroyed, and 
its vermicular motion to aid and accelerate the passage of the 
branchial fluid and its expulsion through the comparatively nar- 
row medial duct paralysed. I may state in corroboration of the 
foregoing observations, that I have removed the posterior hyaline 
process and enlarged the orifice as much as possible, and then 
dropped therein some grains of fine sand to irritate the mem- 
branous spoon-shaped process, when instantly pure water, with- 
out the slightest admixture of fsecal substances, was ejected ; and 
this result was invariable in all and many individuals. I have 
stated that in young transparent specimens an uninterrupted but 
slow action of systole and diastole might be observed, and was 
apparent from the distinct ascent and descent of the water in the 
branchial canal; but this action cannot take place in a merely 
excretory tube ; it can only exist in a circulatory, or inhalant and 
exhalant one. I have carefully dissected the body from the 
animal, ces mouvements alternatifs et mesurés d’inspiration et d’expiration 
du fluide inspiré,” &c. 
On this point that great naturalist is in error, as in Dentalium Tarentinum 
I have with a chronometer showing seconds, repeatedly marked nearly iso- 
chronal inspirations and expirations of the aérating fluid, the two together 
amounting to about twenty-six in a minute. 
