90 SECTIONAL ADDRESSES. 



The piiinitive, free-swimming coelenterate, adopting a fixed habit 

 and becoming attached mouth upwards to soUd rock or stone, gave rise 

 to hydroids, anemones and corals, typical inhabitants of the coastal 

 waters, for the sands and muds at greater depths offered few points 

 of attachment sufficiently stable. 



A Volvox-like colony of simple holozoic flagellates, according to 

 MacBride,^^ commenced to feed upon miscroscopic organisms lying on 

 the sea bottom, and under these circumstances only the cells of the 

 lower half of the colony would be effective feeders. The upper cells, 

 therefore, lost their fiagella and became merely a protective layer, 

 which finally grew downwards outside the others and fixed the colony 

 to the ground. In this way a sponge was formed. The collar cell, 

 so typical of the group, had been developed already by the flagellates, 

 its first inception being perhaps a circle of protoplasmic hairs such 

 as we find in Pedinella. But this adoption of a fixed habit, as it were 

 mouth downwards, did not lead very far, and though there has been 

 much elaboration within the group itself, the sponges have remained 

 an isolated phylum, unable to develop into higher forms. 



It is in a Ctenophore-like ancestor that we find the line of develop- 

 ment to higher animal groups, and this ancestor must have been at 

 one time widely distributed in the seas. Its immediate descendants 

 are familiar to every zoological student in the well-known series of 

 pelagic larval foi'ms. Miiller's larva, taking to the bottom, and in 

 its hunt for food gliding over hard surfaces with its cilia, led to the 

 flatworms; the Pilidium, developing a thread-like body and creeping 

 into cracks and crevices to transfix its prey, gave rise to the nemertines. 

 A Trochophore, burrowing in soft mud and sand, developed a segmented 

 body which gave it later the power of running on these soft surfaces, 

 and became an annelid worm. Another Trochophore, developing a 

 broad, muscular foot, crept on the sand, and afterwards buried itself 

 beneath it as a lamellibranchiate mollusc, or migrated on to harder 

 surfaces as. the gastropod and its allies. Pluteus, Bipinnaria, 

 Auricularia, first fixing, as the crinoids still do, and developing a radial 

 symmetry, afterwards broke free and wandered on the bottom as sea- 

 urchin, star-fish and cucumarian. Tornaria developed into Balano- 

 glossus, whose structure hints to us that the ascidians and vertebrates 

 came from a similar stock. All the phyla thus represented derive 

 directly from the free-swimming Ctenophore-like ancestor, and only 

 one considerable group, the Arthropods, remains unaccounted for. The 

 evolutionary history of an Arthropod is, however, not in doubt. Its 

 marine representatives, the Trilobites and Crustacea, came directly 

 from annelids, which, after their desertion of a pelagic life to burrow 

 in the sea-floor and run along its surface, again took to swimming, and 

 not only stocked the whole mass of the water with a rich and varied 

 life of Copepods, Cladocera and Schizopods, but gave rise to Amphipods, 

 Isopods, and Decapods, groups equally at home when roaming on the 

 bottom or swimming above it. 



Another important addition to the pelagic fauna we should also 



'* Text-books of Embryology. I nvertchrata. London, 1914. 



