182. Mr. W..Clark on Ceecum trachea and C. glabrum. 
marches with great vivacity, carrying its shell sometimes with 
the convexity upwards, resting on the posterior point, or on one 
of the sides, frequently changing one for the other, by suddenly 
withdrawing the head and body, by which action it is thrown on 
the operculum at an elevation of 50° or 60° ; it then turns on the 
side it wishes. 
It thus appears that this minute creature has all the organs of 
the Gasteropoda with entire apertures ; there are some modifica- 
tions of them, and the animal is not spiral ; still in number, qua- 
lity and purpose they are essentially the same as those of this 
large class, and I think it is clear that the genus Cecum must 
be placed with them, in the immediate vicmity of the Rissoe. 
It is necessary to mention that the Dentaliwm imperforatum 
and D. trachea, the types of the genus Cecum, are identical ; 
this fact is I believe generally admitted ; I will however in cor- 
roboration thereof observe, that in the same watch-glass of sea- 
water I carefully examined each of the two forms of this species, 
and their respective organs differed in no respect, except that the 
colour of the buccal mass in C. trachea (Mont.) was somewhat 
paler than that of C. amperforatum, in consequence of its ado- 
lescence. I have made a second examination of the animals of 
both forms with the same result. 
The shell of Cecum imperforatum (Mont.) is never found other- 
wise than adult ; this fact proves that C. trachea is the young 
shell, of which I have seen hundreds of all sizes and gradations 
of arcuation and tapering of the posterior extremities: these 
shells, like some others of the Gasteropoda, particularly those of 
the genera Aporrhais and Turritella, have as a provision of na- 
ture the power to protect their delicate extremities by withdraw- 
ing them from the posterior pomted ends of the shells, some 
chambers of which they plug up; these being deprived of the 
animal, fall off and decay, and it remains uninjured. This is 
the case with Cecum trachea, which probably performs this 
manceuvre more than once, until it arrives at the form C. imper- 
foratum, with its adult constricted orifice, which it never has in 
a young state; and even when the anterior part of the shell 
is broken, the animal always repairs it with a somewhat fuller 
cylinder ; but the new orifice will not be constricted until the 
mutilated shell has arrived at the complete adult state, and it 
is rarely seen in this condition. What is called the posterior 
process of the shell is only one of the testaceous plugs with which 
the animal from time to time closes the posterior extremity. 
As to the specific appellations of ¢rachea and imperforatum, 
though not quite contemporaneous, the more significant one of 
trachea ought, I think, to be adopted, as that of imperforatum is 
obviously improper. 
