8 LOWER INVERTEBRATES. 



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

 objects for examination with a microscope. 



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

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

 that attract more vmiversal 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 fi'om about ^^.j to ^^is 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 Thalassicolla the capsule is very small compared 

 to tlie 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 live in colonies like 

 Thalassicolla pimctata, 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 foimd 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 wliich are supposed to be true nuclei of the capsule. These are of two kinds, — 

 sunple 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 snnilar 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 observed to form the outer boundary of the sphere, as 

 already described in Thalassicolla. 



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 frothy 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 ]iseudopodia 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 \\'ater. They may be 

 retracted and extended. 



