ing the same phylum with the vertebrates (in most 

 classifications) are three small subphyla of inverte- 

 brate chordates built on the same basic body plan 

 but lacking the vertebrated backbone and other in- 

 ternal bones. 



The union of invertebrate animals, on the other 

 hand, is not a natural grouping but merely a con- 

 venient device for talking about at least twenty-eight 

 different phyla — some say more — with as many 

 different basic designs for living. The discrepancy in 

 number of phyla results from differences of opinion 

 as to just what constitutes a body plan distinctive 

 enough to entitle a group to a phylum of its own. 



Classifying animals in neat cubicles labeled with 

 long, resounding names tends to obscure the fact that 

 such names designate phyla of very different size 

 and importance, and that the characteristics used to 

 differentiate the groups are not always of equal mag- 

 nitude. To emphasize these points, the list of phyla 

 given below has apposed to it rough approximations 

 of the number of living species and also a few sub- 

 headings that indicate either deep cleavages or broad 



Subkingdom Protozoa 

 Phylum Protozoa: 30.000 



Subkingdom Parazoa 

 Phylum Porijeni: 4.500 



Subkingdom Metazoa 

 Phylum Coelenlerala: 



9.000 

 Phylum Ctenophora: SO 

 Phylum Mesozoci: 7 

 Phylum Platyhelminthes: 



9.000 

 Phylum Nemertea: 570 

 Phylum Nemutoda: 10.500 

 Phylum Rolifero: 1.200 

 Phylum Gasirotricha: 100 

 Phylum Kinorhyncha: SO 

 Phylum Priapulitia: 6 

 Phylum Ncmatomorpha: 80 

 Phylum Acanlhocephala: 

 400 



Subkingdom Metazoa 



(continued) 

 Phylum Enloprocta: 60 

 Phylum Chaetogiuilho: 30 

 Phylum Hcmichonlalii: 100 

 Phylum Poqonophora: 22 

 Phylum PhoronUhi: 15 

 Phylum Biyozoa: 6.000 

 Phylum Brcichiopodu: 260 

 Phylum Sipunculoidea: 



250 

 Phylum Echiuroidca: 60 

 Phylum Moltuscu: 40.000 

 Phylum AnneUda: 6.000 



Arthropoda (e.x- 

 of in.fecl.^): 65.000 

 EchinodermaUi: 



Phylum 

 elusive 



Phylum 

 5.500 



Phylum 



ChordaUi (exclu- 



sive of vertebrates): 1.320 



bonds. The numbers given here are only tenta- 

 tive and all of them are subject to change as 

 new forms are found, described, and named. Occa- 

 sionally a group even loses a species or two because 

 a specialist finds that two or more named species are 

 really variants of the same species. In practice it is 

 not easy to decide how much variation can be al- 

 lowed within the bounds of a single species or of 

 higher ranks in the classificatory scheme, so that the 

 "lumpers" and the "splitters" among taxonomists 

 often engage in spirited arguments over criteria. If 

 there are difficulties even at the species level, where 

 the specialists are dealing with the more or less natu- 

 ral category that we think of as "a kind of animal" — 

 a man or a dog or a honeybee — it is little wonder 



that the disagreement increases as we approach the 

 larger and more arbitrary groupings. 



It is apparent that the "great divide" in the animal 

 kingdom is not that between vertebrates and inver- 

 tebrates, even though this distinction was first made 

 by Aristotle. He did not use those terms, but mistak- 

 enly divided the animal kingdom into the "enaima" 

 ("bloody animals") equivalent to our vertebrates, 

 and the "anaima" ("bloodless animals") correspond- 

 ing to our modern concept of invertebrates. In his 

 limited experience with invertebrates he did not hap- 

 pen to examine one with red blood, and the colorless 

 blood of invertebrates he did not recognize as blood 

 at all. Though he also recorded that "all sanguineous 

 animals have a backbone," his error of classification 

 stood for more than two thousand years. Then in the 

 early part of the nineteenth century Lamarck used 

 the terms "vertebrate" and "invertebrate," and his 

 fellow Frenchman, the great comparative anatomist 

 Cuvier, made the correct distinction based on the 

 fundamental difference in body plan. 



The really wide gap in the animal kingdom, how- 

 ever, is that which separates the one-celled animals, 

 the Protozoa, from the other phyla which we call 

 the Metazoa because they are many-celled. More 

 will be said of this in the next chapter. Here it is also 

 important to point out the setting apart of the many- 

 celled sponges, or Porifera, as a phylum so different 

 from other Metazoa that we feel it must have had a 

 separate origin from the Protozoa. Among metazo- 

 ans an important distinction sunders the two-layered 

 coelenterates and ctenophores from all the groups 

 which have three well-developed primary embryo- 

 logical layers. This third layer appears between the 

 original two, and it produces those firm and bulky 

 tissues which are so conspicuously lacking in the 

 more fragile kinds of coelenterates. 



The pattern of animal evolution is not a ladder on 

 which the various groups have ascended rung by 

 rung, but a three-dimensional tree with branches 

 that diverge at various levels. For lack of evidence 

 we cannot make out the exact connections of some 

 of the branches. But looking up along the main trunk 

 of the tree we see clearly that it soon splits into two 

 main branches. One of these is the main line of in- 

 vertebrate evolution, which gives rise to the seg- 

 mented worms or annelids; to the two largest inver- 

 tebrate phyla, the mollusks and arthropods; and also 

 to most of the smaller groups. The other main branch 

 is a minor diversion as far as invertebrates go, for it 

 has only one sizeable invertebrate phylum, the Echi- 

 nodermata, which includes the starfishes and their 

 allies. These are sluggish creatures, lacking a head, 

 losing their two-sided symmetry, and possessing the 

 most feeble kind of nervous system. Yet from this 

 stock, man and the vertebrates appear to have come. 



[15 



