OF THE RHIZOPODA IN GENERAL. 15 



proportions, and general arrangement of the pseudopodial extensions ; for, notwithstanding 

 their apparently unrestricted polymorphism, it will be found that Rhizopods present three very 

 distinct types ofpseudopodian conformation, to one or other of whicii they may all be referred, 

 and that the groups thus formed are eminently natural. How intimately related these diver- 

 sities are to those fundamental potentialities of each type which find so little structural ex- 

 pression in this lowest form of animal life, appears from the circumstance that even a particle 

 of protoplasm detached from the general mass of the body will put forth the pseudopodian 

 extensions characteristic of its type ; those of the substance forced out by crushino- the " test" 

 of an Arcella (Plate I, fig. 19), having the broad, lobated form of those of the Amwlja, whilst 

 those of the substance forced out in like manner by crushing the shell of a Polystomella 

 (Plate IV, fig. 12) have the delicate, thread-like character of those of the Foraminifera generally. 



9. In Actinophrys and its allies (Plate I, figs. 1 — 6), which may be considered as constituting 

 the central or typical group of Rhizopods, the pseuclopodia are very numerous, and, when fully 

 extended, are long, slender processes, that gradually taper from base to point, and issue 

 from the body in a radial direction ; they generally remain distinct when they come into 

 mutual contact, never undergoing that complete fusion which is common in those of Forami- 

 nifera; and a slow movement of granules maybe seen to take place along their margin, when 

 the observation is continued for a sufiBcient length of time under high magnifying powers. 

 The difi'erentiation between the central and the peripheral portions of the body is such as to 

 mark out the "endosarc" and the " ectosarc" as, on the whole, distinct from each other, 

 though no definite line of demarcation can be drawn between them. — With the family 

 ActinopJiryna, which includes certain forms that possess a firm envelope over a larger or 

 smaller portion of their surface, there seem to be associated, by their more or less complete 

 conformity in the foregoing characters, the Acanthometrina, in which the body is supported by 

 a regular framework of radiating siliceous spicules, the Polycystina, in which it is more or less 

 completely encased in a siliceous "test," and the Thalassicollina, which, whilst apparently 

 sometimes simple, are generally composed of aggregations of more or less fully difi"erentiated 

 cells, supported upon a framework of siliceous spicules. This last family obviously establishes 

 the transition between the typical Rhizopods and Sponges. The four families now enumerated 

 seem to constitute an eminently natural order, to which the designation Radiolaria 

 proposed by Midler,* (lxviii) is very appropriate. 



10. In Amwba and its allies, which diverge from the typical Rhizopods in the direction 

 of Gregarinida and Infusoria (both of which Classes contain forms that are scarcely 

 distinguishable from it), the pseudopodial expansions are few, short, broad, and rounded (Plate 

 I, figs. 1.5 — 20), seeming rather like lobate extensions of the body itself, than appendages pro- 



* The Rliizopoda Radiolaria of Miiller did uot include the Actinopliryna ; and he separated them 

 iuto two sub-orders, according as they are Simple or Composite — a di.stinction which does not appear 

 to the author at, all better founded than that according *to which the Foraminifera are primarilv 

 divided into Monotlialamous and Pohjtltalamous (see If 52). He is glad to find that the views of 

 MM. Claparede and Lachmann, on this point, seem to be in accordance with his own ; no notice being 

 taken, in their classification of Rliizopoda (xxv, p. 434), of the families Spliarozoa and Collospltoercs 

 established by Miiller for the composite forms of ThalassicolUe. 



