ORGANIC FORM AND ARCHITECTURE 671 



Echinoderms are complex: the larvae may be bilateral, like the 

 "painter's easel" Pluteus; the adults may be radial superficially and 

 in some measure internally; but if in strictness we insist that a sea- 

 urchin and a starfish are bilateral (considering the madreporic 

 plate and so forth), and if we further recognise the obvious bila- 

 terality of a heart-urchin (e.g. Echinocardium and Spatangus), we 

 have to notice at the same time the curious fact that the plane of 

 the adult's bilateral symmetry is not the same as the larva's! 



It may be noted that while the asymmetry is usually between 

 right and left sides, this is not the only possibility. Thus the bilat- 

 erally symmetrical Lamp-shells or Brachiopods — which have a 

 slight superficial resemblance to the bilaterally symmetrical bivalve 

 molluscs, though in no way related to them — often show a marked 

 inequality between the dorsal and the ventral valves, contrasted in 

 their position with the right and left valves of Lamellibranch 

 molluscs. 



Many free-swimming animals, from Paramoecium to larval Asci- 

 dians, move in a helicoid spiral, as man does in a fog or when swim- 

 ming blindfolded. Though this is not due to any obvious inequality, 

 e.g. of legs, it is probably correlated with some deep and subtle 

 asymmetry. 



ORGANS. — ^Any well-defined part of an organism that has a 

 dominant function and some measure of independence may be 

 called an organ, as in the case of brain and heart. But many an 

 organ, such as leaf and liver, has numerous functions. There is 

 something to be said for using the term organella for a specialised 

 part of a unicellular animal or plant, such as a locomotor fiagellum 

 or a contractile vacuole or a pigment-spot. 



As to historical emergence, it can hardly be said that Sponges or 

 Porifera have organs; they remain at the level of histonal or tissue 

 animals. The first organs in the Animal Kingdom are seen in the 

 Phylum Coelentera, where one may find not only an enteric 

 cavity (or food-canal), which might be claimed for sponges, but in 

 some cases, like sea-anemones and true jelly-fishes, a specialised 

 gullet and digestive filaments. Reproductive organs, sense-organs, 

 nerve-ganglia, and other specialised parts soon make their appear- 

 ance. After the establishment of a food-cavity — the "archenteron" 

 of the gastrula embryo in the individual, the enteron of the ancestral 

 gastraea for the race— the first organs to appear were probably the 

 reproductive organs or gonads. Thus a very simple animal like 

 Microhydra has a food canal and gonads, but no other organs at all, 

 not even tentacles! If we rank the food-canal as the first organ, we 

 may put the gonads second. It has been suggested, however, and 

 there is interest in the speculation, that if multicellular evolution 

 started from a hollow ball of cells as in Volvox, the central cavity 



