70 SECTIONAL ADDRESSES. 



Grant named the group Porifera 36 from the pores of their external surface, 

 through which all water enters to the collar-cells. We know now that in 

 most sponges these pores represent only the most exterior of two, three or 

 more filters, interposed to prevent access of larger bodies than those which 

 the collar-cells ingest. There is no sponge into which the sponge current 

 can carry a body larger than a frog's blood-corpuscle, and in most the 

 entrance pores of the flagellate chambers will not admit a particle of 

 one-third of this diameter. 



Stomached animals are formed with a cavity into which many cells 

 pour a secretion, which thus digests up to fragmentation portions of organic 

 matter too large to be ingested by any single cell. In the early Flagellate 

 Ocean there were no particles of organic matter too large to be digested by 

 a single cell, and therefore stomached animals did not arise until the) r had 

 multicellular algae and sponges on which to feed. The sponges are therefore the 

 more ancient group, and have had little need for change, except for defence 

 against animals. Few of the changes which have taken place in mollusc, 

 crustacean, or vertebrate, affect their life ; except as the abundant repro- 

 ductive products of the Metazoa provide more food for the Porifera. The 

 secondarily assumed rnicrophagous habits of Lamellibranchs and Tunicates 

 do not prevent the sponges, with their longer history, from flourishing on 

 an oyster-farm and damaging the oysters. When we watch the currents 

 of a sponge, or under the microscope watch its collar-cells feeding, we are 

 seeing, unchanged, what took place in the seas of the Pre-Cambrian more 

 than 1000 million years ago, when the inhabitants of the ocean were 

 flagellates, low algae, and sponges. 



How did animals come ? Embryologists tell us of the blastula — ■ 

 a common form of motile spore-bearer among flagellates, never, so far as 

 I know, observed to feed except by photosynthesis. They tell us of a 

 gastrula, but the laws of viscous motion make it clear that the free-swim- 

 ming gastrulae we observe as larvae could never earn their own living, 

 since the stream-lines would carry every particle of food outside the cone 

 of dead water which is dragged behind the gastrula mouth. Creeping 

 planulae or gastrulae might pick things up, but as a predatory organism 

 a free-swimming planula would seem a most ineffective and unarmed 

 buccaneer, 37 and it is not surprising that in nature no such creature has 

 ever been known as an adult, and no planula or gastrula larva has ever 

 been recorded to have taken into its endoderm a single particle of food. 



In the embryology of sponges I am convinced that the blastula, gas- 

 trula and planula have no historical significance, other than the fact that 

 flagellates are in the habit of distributing themselves by flagellate aggre- 

 gates, often of the blastosphere form. For the rest, the amphiblastula of 

 Sycon and the planula of Axinella are merely convenient ways of arranging 

 in the motile aggregate the segregated elements of the future tissues. The 

 embryology of animals is held by those who study it to be more significant 



36 Grant called them first Porophora (1825, Edin.Phil. Journ.), then Poriphera (' Out- 

 lines of Comparative Anatomy,' 1835, p. 5), and Porifera in 1855, Todd's Cyclopaedia, 

 ■fide Vosmaer (Bronn, p. 52). Hogg objected (1839, p. 399 footnote) that he had used 

 the word Porifera first in 1827 for an order of corals {Cellepora, &c). 



37 So also E. W. MacBride : 'Text-book of Embryology,' p. 99. London, 1914, 

 Macmillan. 



