M. E. Peuard's Notes on some Ileliozoa. 147 



Now most authors, who, it must be said, seem to have the 

 idea only of a continuous membrane where there are in reality- 

 only free scales plunged in a mucilage, have regarded either 

 this membrane or the narrow clear zone of plasma which lies 

 immediately within it as a sort of exudation of the central 

 plasma of no particular importance ; and as, on the other 

 hand, this central plasma is really often seen split into two 

 concentric zones, the two latter have, in the Acanthocystidae. 

 been denominated the ecto- and endosarc. 



In my opinion we have here a confusion. In fact I have 

 been able to ascertain that the skeletal mucilaginous zone is 

 perfectly active, and behaves physiologically — for example, 

 during the capture of prey — like the vacuolized ectosarc of 

 Actinophrys. I should therefore be inclined to regard it as 

 the true ectosarc. It is true that this opinion is open to 

 dispute ; but as I have referred to it at greater length else- 

 where *, I shall not dwell upon it here ; moreover the 

 sequel of this communication will contain some explanations 

 of this point. 



Let us return to the skeleton properly so called, to the 

 siliceous spicules, and take as an example one of the largest 

 species, Acanthocystis turfacea, Carter, which is best fitted 

 for observation. 



The skeleton of a typical and adult Acanthocystis turfacea 

 is composed of siliceous elements of three forms : — 



a. Of thick, very short, tangential scales, giving rise by 

 their combination one after the other to the appearance of a 

 continuous membrane ; 



b. Of large radial spicules, bifurcate at the apex, and ter- 

 minated like nail-heads at the base, nearly equalling in length 

 the diameter of the animal itself; and 



c. Of smaller radial spicules, exceedingly fine, widely bifur- 

 cated at the apex, intercalated among the large spicules. 



The structure of these spicules is not well known, and 

 therefore I may venture to dwell upon them for a moment. 

 Having had the opportunity of observing a great number of 

 individuals of different ages, I have in the first place ascer- 

 tained certain points, which may be summed up as follows: — 



a. The long spicules of Acanthocystis turfacea are thicker, 

 more definite, and longer in proportion as the animal is older. 



b. In the young we see only the stem of the spicule, which 

 is tine and not well defined; the nail-head- of the base and 

 the fork at the apex are not visible (do not exist ?) . 



* ' Archives cle Biologie,' tome ix. (188D). 



