68 LOWER INVERTEBRATES. 



existence of a cloacal outlet does not necessarily indicate the presence of an individual. 

 We must regard the whole mass which springs from one base as being an individual, 

 while the buds or branches which may arise from it are not branches, but may be 

 regarded as the prototypes of true buds and branches of colonial animals in other 

 divisions of the animal kingdom. They resemble the branches of other colonies in 

 aspect, and arise from unequal growth of parts in a more or less symmetrical way, and 

 may have any outline essential to the equilibrium of the form, but are no more indi- 

 viduals than are the arms and legs of a human being. 



The whole mass is the individual, and the fact that it has a branching gastrovas- 

 cular system is accounted for by the budding of the coelomatic cavity just as the gas- 

 trovascular system of the Hydrozoa and the water system in echinoderms is formed by 

 the prolongation or budding of the walls of the gastric cavity of the larval forms. In 

 fact the similarity of these parts in the Coelenterata and Echinodermata indicate to us 

 that the sponges present a much more primitive condition of the gastrovascular sys- 

 tem than do any of the higher types. In the echinoderms, the system becomes sepa^ 

 rated into the gastric cavity and the system of water tubes in the early stages ; in the 

 Hydrozoa and Ctenophora, the two remain in connection and a true water system is 

 not developed. In the sponges there are two systems, the supply or water system and 

 the cloacal or gastric system, and these two together make a complete gasti'ovascular sys- 

 tem which, however, is more primitive than either of the other types, combining both 

 the gastric and the water systems in a double set of inter-communicating canals. It is 

 difficult to explain the similarities of the water systems among these animals on any 

 other grounds, and this view enables vis to throw some light upon the similarities of the 

 coelomatic cavity. This cavity is merely the primitive hollow of the body of the 

 embryo, and in many of the lower forms, as the Hydrozoa, it is the digestive cavity, 

 the cells being modified for assimilative purposes. This is only the next stage above the 

 cellular mode of digestion in which each cell performs this function as in the ampullae 

 of the sponge, and it is an adaptive change both in the structure of the cells and their 

 function. 



If our view of the affinities of the sponges is correct, this cavity in the Ascones is 

 directly derived from the communal inlet and outlet of some colonial form of Protozoa, 

 and the water system must have arisen subsequently in response to the budding of the 

 coelomatic cavity, and the need of special sources of supply for each bud with its 

 ampullae. The correlations in the structure of the feeding cells of sponges and the 

 aspect and similar functions of the cells which form the tissues of higher animals show 

 that not only the sponges, but all the Metazoa, however highly individualized, must be 

 regarded as aggregates or colonies in which each cell represents a zoon of the Protozoa, 

 and which has derived its structure by inheritance from an ancestral protozoon. That 

 is to say, there is such a phenomenon as the inheritance by the single cells of a meta- 

 zoon of the peculiarities and even the tendencies of the independent individualized 

 protozoon, and from this results the communal characteristics of the metazoon which 

 appears to be, but in reality is not, a simple individual. The only simple individual in 

 the animal kingdom is the single unicellular protozoon, or a single cell from the tissues 

 of the Metazoa. 



This view, though for a time overwhelmed with ridicule, has of late j'ears obtained 

 a quite general acceptance. It dates back to an inspiration of Oken in 1805. The 

 transitions by which it could have taken place have never been satisfactorily stated nor 

 can we here do anything more than add another step towards a final solution. It is now 



