Mr. W. Clark on the Animal of Dentalium Tarentinum. 325 



only to consider that the water in this genus flows to the 

 branchiae 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 

 branchiae 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 striae, 

 in the hollows 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 oeconomy ; 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 faecal 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 mesur6s d'inspiration et d'expiration 

 du fluide inspire," &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 aerating fluid, the two together 

 amounting to about twenty-six in a minute. 



