Mineral Deposit in the Rhizopods and Sponges. 81 



In the Acanthometrina which belong to another order, namely, 

 the Protodermata, the plan of siliceous deposit is nevertheless 

 essentially the same — the elongated spines (acanthostypes) never 

 being tubular, as erroneously asserted by Professor Miiller. The 

 appearance of tubularity in these organisms, as in the Polycystina, 

 is an optical illusion engendered by the longitudinal ribs of 

 which the acanthostypes may be said to be made up*. 



In the Thalassicollidse, which, together with the Dictyochidae, 

 are placed by me in the same order as the Acanthometrina, the 

 plan of deposit is for the first time modified, but only to the extent 

 of taking place in two directions from the axis of each spicule. 

 In other words, the spicule is deposited within the sarcode en- 

 tirely, but not in previously existing cavities or around stolons. 

 Hence the silex is secreted only from within outwards. 



Lastly we arrive at the Dictyochidae, a group I have found it 

 necessary to set apart as a distinct family, owing to the fact 

 of their presenting the solitary example of true tubular forma- 

 tion amongst the whole of the Rhizopods. In the organization 

 of their soft parts they are closely allied to the Acanthometrina 

 and Thalassicollidse ; whilst the tubularity of their siliceous 

 framework, and its formation of two separate isometrical portions, 

 at once stamp this family as the true connecting link between 

 the llhizopods and Sponges. It is a singular fact that up- 

 wards of twenty varieties of these very common organisms have 

 been described as distinct species on characters of no higher 

 import than the number of spines or angles presented by the 

 siliceous framework ; and that some have actually been described 

 and figured as seen in a living condition with half of the internal 

 skeleton deficient I 



It only remains for me to add that in other genera of the 

 Foraminifera than those referred to by Professor Schultze {as, 

 for instance, in Globigerina) , and likewise in some of the Poly- 

 cystina [Haliomma), specimens are not unfrequent in which the 

 chambers are more or less choked up with entozootic sponge- 

 growth ; whilst the chambers of Globigerina are at times filled 

 with efi^ete frustules of a free-floating pelagic surface Diatom, 

 namely, Chatoceros. 



Hence, assuming the order of deposit of the mineral matter 

 of all these structures to be constant amongst the members of 

 the same family, the facts now advanced furnish the fullest 



* In the * Annals ' for October 1863, Mr, Carter describes, under the 

 name of Acanthocystis turfacea, an organism recently found by him in De- 

 vonshire, which he refers to the order Echinocystidia (Echinocystida?) of 

 MM. Claparede and Lachmann. The spines in this form are said to be 

 hollow ; but, for reasons above given, this would at once remove it from 

 Acunthometra, and indicate, in this respect, its close affinity to Acineta. 



Ann. ^ Mag, N. Hist. .Ser. 3. Vol. xiii. 6 



