178 MORPHOLOGICAL DEVELOPMENT. 



ascend to aggregates in which, we have seen reason to suspect 

 a higher degree of composition. Good examples of the con- 

 nexions between forms and forces occur in this group. 



Among the lower annuloid types, 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 extremi- 

 ties, unlike above and below, but having its two sides alike. 



The Echinodermata furnish us with instructive illustrations 

 instructive because among types that are nearly allied, we 

 meet with wide deviations of form answering to marked con- 

 trasts in the relations to the environment. The facts fall 

 into four groups. The Crinoidea, once so abundant 



and now so rare, present a radial symmetry answering to 

 an incidence of forces that is equal on every side. In the 

 general attitudes of their parts towards surrounding actions, 

 they are like uniaxial plants or like polypes ; and show, as 

 they do, marked differences between the attached ends and 

 the free ends, along with even distributions of parts all round 

 their axes. In the Ophiuridea, proved to be near 



akin to the Crinoids, and in the Star-fishes, we have radial 

 symmetry co-existing with very different habits ; but habits 

 which nevertheless account for the maintenance of the form. 

 Holding on to rocks and weeds by its simple or branched 

 arms, or by the suckers borne on the under surface of its 

 rays, one of these creatures moves about not always with one 

 side foremost, but with any side foremost. Consequently, 

 averaging its movements, its arms or rays are equally af- 

 fected, and therefore remain the same on all sides. On 

 watching the ways of the common Sea-urchin, we are 

 similarly furnished with an explanation of its spherical, or 



