214 GEOLOGICAL BIOLOGY. 



elaboration seen in the true Ccelenterata, but rather con- 

 stituting a colony of Protozoa-like individuals. The true 

 Ccelenterata, as illustrated by the corals or Anthozoa, are 

 elaborated a step further; in their gastro-vascular cavity a 

 certain polarity (5) of the body is differentiated, of which the 

 mouth is the centre, the polarity is expressed functionally in 

 the direction of the currents inward and outward through the 

 mouth ; in the motor system special (6) motory organs are 

 developed, radiating from and surrounding the mouth as 

 tentacles (6a), and the whole of the body, in the higher forms, 

 also expresses this radial arrangement of parts into compart- 

 ments (6$), called mesenteries. This radial differentiation is 

 indefinite in the earliest forms, but there are two modes of 

 division that are well expressed later, seen in the tetracoralla 

 (7*7) and the hexacoralla (7^). In the Cambrian only the four- 

 parted type (Tetracoralla) was specialized. These constitute 

 the Rugosa; also, the Medusa (8) appeared in the Cambrian, 

 according to Walcott. In the Mesodermal layer are differen- 

 tiated both muscular (9) and skeletal (10) tissues, which take 

 the radiate form of the mesenteries, and in the living forms 

 there is a differentiation of the sex (n) a differentiation we 

 have all reason to believe was existent in Cambrian time. In 

 the ectodermal or outer layer of the body there is differen- 

 tiation of a set of cells for offensive and defensive action upon 

 other organisms; these are the thread cells (12), which are 

 used offensively (12*2), probably to benumb their prey and 

 thus aid in the attainment of food, and as defensive (i2#), in 

 the way of protecting themselves from attack of larger ani- 

 mals which might seek them for food. 



There is no certain differentiation of sense or nervous or- 

 gans in the Coelenterata, and the above points are about all 

 that can be said certainly to apply to the organisms referred 

 to the Coelenteraca for the Cambrian era. 



The branch Echinodermata also was present in the Cam- 

 brian. In them the body presents the radiate (13) type 

 of structure in the adult, but the parts are normally five 

 (13^), and there is more or less distinct bilateral sym- 

 metry (14) exhibited by them in the adult form generally, 

 or only in the embryonic form in some of the living types* 



