182 Mr. W. Clark on Caecum 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 Ccecum must 

 be placed with them, in the immediate vicinity of the Rissoce. 



It is necessary to mention that the Dentalium imperforatum 

 and D. trachea, the types of the genus Cacum, 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. imperforatum, 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 Ccecum 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 pointed 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 Ccecum trachea, which probably performs this 

 manoeuvre 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 trachea 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. 



