6 THE INVERTEBRATA 



zooids of a colony are usually all alike, but differentiation may exist 

 between them, in that certain of them are specialized for the produc- 

 tion of new colonies, which is not performed by the other zooids 

 (various volvocina, Figs. 3, 46; etc.). Colonies arise by the division of 

 a single primary zooid, whose fission is not carried to completion, so 

 that its products do not entirely separate. Their origin is therefore 

 usually said to be a form of asexual reproduction. It may, however, 

 also be looked upon from another point of view, as the repetition, 

 within a continuous mass of protoplasm, of the nucleus and the other 

 organs coincidentally. In this aspect, the colony is seen to have 

 features in common with other multinucleate conditions of protozoa, 

 such as (i) that oi Hexamitus (Fig. 4), etc., in which a unitary body has 



sottiAtic cdU 



Fig. 3. Colonial volvocina. a, Eudorina, a spherical motile colony of thirty- 

 two similar zooids all capable of division, b, Pleodorina illinoiensis, a spherical 

 motile colony consisting of thirty-two zooids, of which four at one end of the 

 colony constitute a "soma", which dies when the other twenty-eight zooids 

 divide, c, Pleodorina californica. The "somatic cells" constitute about half 

 the colony. After West and Fritsch. 



two similar sets of organs, one on each side of the body, or several sets, 

 with a nucleus assigned to each, (2) that of Polykrikos (Fig. 40 B), etc., 

 in which there are several nuclei, and several sets of the other organs 

 of the body, but the repetition (merism) of the nuclei and that of the 

 other organs do not correspond, and (3) that of Opalina (Fig. 5), 

 Actinosphaerium (Fig. 33), etc., in which there are numerous nuclei, 

 but only one set of the other organs of the body. Multinucleate 

 masses of protoplasm are known as syncytia. Syncytia which, like 

 those cited above, arise by the division of an original nucleus in the 

 mass of protoplasm are known as symplasts. An entirely different kind 

 of syncytium arises by the union of uninucleate individuals, whose 

 nuclei remain distinct in the resulting body. Such syncytia are known 



