1868. | Zoology. 271 
mesenteric filaments, there are only two, supported upon two con- 
secutive septa. The cavity of the body of these individuals is 
always in communication with that of the sexual individuals ; but 
the mode in which it is effected varies in the genera. Among the 
coral forms which exhibit this interesting polymorphism are the 
sea-pens, Virgularia and Pennatula. With the exception of Sar- 
cophyton, Professor Koélliker has not observed this dimorphism in 
any of the Aleyonide or Gorgonide. . 
Do Molluscs bore by Acids?—There are some naturalists, de- 
voted conchologists, who lose their temper whenever this question 
is asked, considering that it has been completely settled that acids 
have nothing to do with the holes made in limestones by any 
animals whatever, because a Pholas has been found to have bored 
into a piece of gneiss. Ten years ago M. Troschel, an eminent 
observer, detected a considerable quantity of free sulphuric acid in 
the saliva of a Gasteropod — Dolium galea. MM. de Luca and 
Panceri have lately resumed the investigation of this subject, and 
have found between three and three-and-a-half per cent. of free 
sulphuric anhydride in the saliva of this animal; they did not find 
any hydrochloric acid. They have detected sulphuric acid also in 
four species of Tvitoniwm, in a Cassis, a Cassidaria, two Murices, 
and an Aplysia. The part played by these acids and the carbonic 
acid which they also obtained in large quantities is still obscure ; 
but there can be no doubt whatever that were one of these mol- 
luses to eject its fluid on a piece of carbonate of lime (either the 
shell of its prey or the rock on which it lives), that carbonate of 
lime would be dissolved, destroyed, and eroded. The detection 
of an acid, and the ascertaining of its chemical nature, is interesting 
in connection with the discovery by Mr. Ray Lankester of an acid 
excretion from the body of a species of Leucodore, and of Sabella 
which bore cavities in limestone rocks. 
The Nature of Monads.—It appears from the researches of the 
last five years, that many forms of minute life, which at one time 
were regarded as forming families or groups by themselves, are 
really the common modification or stage of existence through which 
many and widely differing organisms pass. At the same time it 
appears probable that this common stage of existence is represented 
in a few cases by adult forms. The cases to which we allude are, 
firstly, Monas forms; secondly, Amaba forms ; and thirdly, Acineta 
forms. There is little doubt that Amoeba is an adult and distinct 
form, as also is Acineta; at the same time, Cienkowski has shown 
that certain minute vegeto-animal organisms pass through first a 
Monas stage, then become small actively creeping Amcebx, and 
finally, becoming encysted in cellulose and tinted with chlorophyll, 
break up into spores, which again become Monas forms; whilst 
De Barry has described a Monad and Ameeba stage in the Myxomy- 
