Exercises XII and XIII 



THE ARRAY OF LIVING ORGANISMS 63 



which develop in one part of the tiny ganieto- 

 phyte, can swim to the eggs, which develop in 

 another part of the gametophyte, at the bottom 

 of a cleft. 



The conifers have made further steps in 

 adaptation to land life. They have developed 

 two types of spores, which give rise respectively 

 to male and female gametophytes. The male 

 gametophyte, now a pollen grain, is dispersed by 

 the wind or by insects, so eliminating the need 

 for water. The female gametophyte is entirely 

 parasitic, living always within the tissues of the 

 sporophyte. On fertilization of the egg by the 

 sperm delivered by a pollen grain, it yields a 

 sporophyte embryo, which, provided with food 

 and a protective coat, is the seed. On being 

 planted, this develops into the mature sporo- 

 phyte. 



In the flowering plants (angiosperms) the re- 

 productive systems achieve further refinement. 

 Stems and leaves are modified to form flowers, 

 which contain the gametophyte generation. (We 

 shall study flower structures in detail next 

 semester, so we need not go into them deeply 

 now.) The gaily colored flowers with their 

 perfumes and nectars attract insects and birds, 

 which willy-nilly transport pollen from one 

 flower to another, ensuring efficient fertilization. 

 Fertilization of the egg within the ovary of a 

 flower leads, as in conifers, to the growth of an 

 embryo sporophyte, which, with its surrounding 

 tissues and protective coat, constitutes the seed. 

 The angiosperms, however, go one step further 

 than the conifers, enclosing the seed in a fruit, 

 which develops from tissues of the flower. The 

 fruits may be eaten by animals, which dissemin- 

 ate the seeds over the countryside. 



The angiosperms are the most complex and 

 successful land plants. They include about | of 

 all living plant species. They divide into the so- 

 called monocots and dicots on a rather trivial 

 basis, whether the cotyledons, the food-con- 

 taining, leaflike structures within the seeds, are 

 single (as in a corn seed) or double (as in a 

 peanut). The monocots include the grasses, and 

 several minor groups, palms, lilies, onions, and 

 orchids. The dicots are almost everything else. 



THE ANIMAL KINGDOM 



(Readings: S.P.T., Chapters 22 and 23. Villee, pp. 

 195-207; Chapters 14 and 15. Weisz, pp. 667-702. 

 An excellent additional source that does almost the 

 whole job in a couple of hours of pleasant reading is 

 the Golden Science Guide, Zoology, by H. S. Zim, 

 H. I. Fisher, and R. W. Burnett. Also, see an excel- 

 lent discussion in Weisz, Chapters 29 and 31, and 

 the fine pictures in Ralph Buchsbaum's Animals 

 Without Backbones, University of Chicago Press, 

 rev. ed., 1948.) 



Principal groups and numbers of species 



PHYLUM CHORDATA (50,000) 



Subphylum: vertebrates 

 Classes: mammals 

 birds 

 reptiles 

 amphibia 

 bony fishes 

 cartilaginous fishes 

 placoderms (extinct, armored, 



jawed fishes) 

 jawless fishes (Cyclostomes) 

 Subphyla (3) of protochordates (Amphioxus, 

 acorn worms, tunicates) 

 PHYLUM ECHiNODERMATA, "spiny-skinncd" 



(6000) 

 PHYLUM ARTHROPODA, "jointed-legs" (1,000,000) 

 Classes: insects 



arachnids (spiders, horseshoe 



crab) 

 Crustacea (crabs, lobsters, bar- 

 nacles) 



PHYLUM MOLLUSC A (100,000) 



Classes: gastropods (snails, slugs, whelks) 

 pelecypods (clams, mussels) 

 cephalopods (squid, octopus, 

 nautilus) 

 PHYLUM ANNELIDA, segmented worms (10,000) 

 PHYLUM NEMATODA, roundworms (10,000) 



PHYLUM PLATYHELMINTHES, flatworms (10,000) 



PHYLUM COELENTERATA, corals, jcllyfishes, hydro- 

 zoa (10,000) 



PHYLUM PORiFERA, sponges (15,000) 



