PROTOZOA: FORAMINIFERA. 55 



foramina. In these forms {e.g., Miliola, fig. 4, b) the pseudo- 

 podia are emitted solely from the mouth of the last-formed 

 segment of the shell. The vitreous shell is transparent and 

 glassy in texture, and its walls are perforated by numerous 

 pseudopodial apertures. The arenaceous shell is, properly 

 speaking, not a true shell secreted by the animal, since it is 

 simply composed of particles of sand united together by some 

 unknown cement. Its walls may or may not be traversed by 

 pseudopodial foramina. 



As regards the form of the shell, the Fora^ninifera may be 

 conveniently, though arbitrarily, divided into two sections : 

 the Monothalamia and the Polythalamia. In the first of these 

 sections (fig. 4, a), comprising the so-called "simple" or "uni- 

 locular " Foraminifera, the shell consists of a single chamber, 

 and the animal is, in fact, nothing more than a little mass 01 

 sarcode enveloped in a calcareous covering. Lagena, with its 

 beautiful flask-shaped shell, may be taken as the type of this 

 division. Another well-known unilocular form is Eniosoknia, 

 which is like Lagena in shape, but has the tubular neck reversed, 

 so as to be inserted into the interior of the test. In the 

 Polythalamia., or " multilocular " Foraminifera, the 'shell is 

 composed of many chambers separated from one another by 

 divisional walls or "septa" (fig. 4, c, d, c), each of which is per- 

 forated by one or more openings, " septal apertures," by means 

 of which the sarcode occupying the different chambers is 

 united into a continuous and organic whole, the connecting 

 bands being called "stolons." Complex as their structure 

 often is, the compound Foraminifera are, nevertheless, formed 

 by a process of continuous gemmation or budding from a 

 single " primordial segment " in every respect identical with 

 the permanent condition of a simple species. They commence 

 their existence, therefore, as Monothalamia, and are converted 

 into Polythalamia merely by a process of " vegetative " or " irre- 

 lative repetition." As their development proceeds, the primitive 

 mass of sarcode, or "primordial segment," throws out fresh 

 segments in the form of buds according to a determinate law; 

 and it is upon the direction in which these segments are 

 evolved that the ultimate form of the shell depends. The 

 more important variations in this respect are as follows : — If 

 the additional segm.ents are added to the primordial chamber 

 in a linear series, so as to form a straight or slightly curved 

 line, we obtain respectively a Nodosaria (fig. 4, d, e) or a £>en- 

 talina. When the new chambers are added in a spiral direction, 

 each being a little larger than the one which preceded it, and 

 the coils of the spiral lying in one plane, then we get the " nauti- 



