THE GENERAL SHAPES OF ANIMALS. 195 



irregularly they are irregularly placed on surfaces inclined 

 in all directions. Merely noting that this asymmetrical 

 distribution of the united individuals is explained by the 

 absence of definiteness in the relations of the aggregate to 

 incident forces, it concerns us chiefly to observe that the 

 united individuals severally exemplify the same truth as do 

 similarly-united individuals among the Ccelenterata. Averag- 

 ing the members of each society, the ciliated tentacles they 

 protrude are similarly related to prey on all sides; and 

 therefore remain the same on all sides. This distribution of 

 tentacles is not, however, without exception. Among the 

 fresh-water Polyzoa there are some genera, as Plumatella and 

 Crystatella, in which the arrangement of these parts is very 

 decidedly bilateral. Some species of them show us such 

 relations of the individuals to one another and to their sur- 

 face of attachment, as give a clue to the modification; but 

 in other species the meaning of this deviation from the 

 radial type is not obvious. 



§ 249. In the Platyhelminthes good examples of the con- 

 nexions between forms and forces occur. The Planaria 

 exemplifies the single bilateral symmetry which, even in 

 very inferior forms, accompanies the habit of moving in one 

 direction over a solid surface. Humbly organized as are 

 these creatures and their allies the Nemertidce, we see in 

 them, just as clearly as in the highest animals, that where 

 the movements subject the body to different forces at its two 

 ends, different forces on its under and upper surfaces, and 

 like forces along its two sides, there arises a corresponding 

 form, unlike at its extremities, unlike above and below, but 

 having its two sides alike. 



tThe Echinodermata furnish us with instructive illustra- 

 tions — instructive because among types that are nearly allied, 

 cc 



e meet with wide deviations of form answering to marked 

 contrasts in the relations to the environment. The facts fall 

 into four groups. The Crinoidea, once so abundant 



