8 LOWER INVERTEBRATES. 



when properly cleaned and prepared for exhibition, afford some of the most beautiful 

 objects for examination with a microscope. 



The Polycystina especially, which have an external skeleton of clear, glassy silica, 

 are to be found in every collection of microscopic objects, and there are few specimens 

 that attract more universal admiration for beauty and regularity of form. 



Before describing some of the more important representatives of this group, a few 

 words should be said concerning their general characteristics. The single animals or 

 zooids vary in size from about 5^^ to ^ of an inch, or even more, in diameter. They 

 are usually spherical, but they may be cylindrical, discoidal, or of other shapes. The 

 sarcode within and without the capsule is continuous through the pores of the chitinous 

 membrane which surrounds it. In ThalassicoUa the capsule is very small compared 

 to the size of the animal, but usually, especially in the solitary forms, the capsule is 

 relatively very large, sometimes having only an exceedingly thin layer of extra capsular 

 sarcode about it. The tendency of such simple forms of life is to Uve in colonies like 

 ThalassicoUa punctata, in which the capsules and the investing sarcode have already 

 been described as cells imbedded in the gelatinous connecting mass. The capsules 

 vary in size from 2 mm. down to .025 mm. 



The sarcode contains vesicles or alveoli, which may be found both within and with- 

 out the capsule ; but no regularly contracting vesicle, such as is found in the Heliozoa, 

 has been observed. 



Within the capsule are found peculiar structures which have been termed nuclei, 

 and which are supposed to be true nuclei of the capsule. These are of two kinds, — 

 simple and complex. The simple nuclei measure from .008 mm. to .015 mm. in diame- 

 ter. They are perfectly homogeneous in appearance, and may exist in great numbers 

 in a single capsule, almost filling it in fact, or they may be few, or even quite absent 

 when a complex nucleus is present. They have no investing membrane. The complex 

 nucleus is a multi-globular vesicle with a membranous covering similar to that of the 

 capsule itself, but more delicate. It is possible that the simple nuclei are developed 

 within it. The complex nucleus is also designated as the "nuclear vesicle." It is 

 characteristic of certain forms of Radiolaria. 



The sarcode of the capsule may be colorless, or it may be distinctly colored, red, 

 brown, and yellow being the usual colors. Examination with high powers of the micro- 

 scope shows the coloring matter in the form of minute vesicles. There are also found 

 in the sarcode globules of oil or fatty matters, and sometimes concretions, crystals, and 

 other structures that may be nothing but remains of food. The external sarcode is not 

 protected by any definite enveloping membrane, but a clear, gelatinous, more or less 

 firm layer of the sarcode may be o]jserved to form the outer boundary of the sphere, as 

 already described in ThalassicoUa. 



The sarcode of the central capsule is continuous with the external sarcode through 

 the pores of the dividing membrane. The extra capsular sarcode is usually fi-othy in 

 appearance owing to the presence of clear spaces, — vacuoles or alveoli. These alveoli 

 usually increase in size from without inwards, being largest and most numerous near the 

 capsule. The outer alveoli have been observed to disappear at times and to form again. 



The pseudopodia of the Radiolaria resemble those of the Heliozoa, being more or less 

 persistent and not very flexible. In some species they branch and anastomose slightly. 

 They originate from the deepest part of the external sarcode, pass between the alveoli 

 and through the gelatinous investment into the surrounding water. They may be 

 retracted and extended. 



