When the Protozoa are considered members of the 

 Kingdom Protista, the animal kingdom is easily sepa- 

 rated from the plant kingdom. Animals lack chloro- 

 phyll, cells containing a cell wall, plastids, and a cen- 

 tral vacuole. However, some similarity of animals 

 and plants is implied in the chemical resemblance of 

 the animal blood pigment, hemoglobin, to chloro- 

 phyll. 



Many other differences often exist between plants 

 and animals. Only animals have the following as 

 well-developed tendencies; mobility, holozoic nutri- 

 tion, fixed structure, marked responses to stimuli, 

 body fluids resembling sea water, and structures and 

 functions that tend to maintain a constant internal 

 body environment. In addition, animals almost in- 

 variably have a haplobiontic, adult diploid, life cycle. 

 However, an alternation of generations in which one 

 adult is produced by asexual reproduction also 

 occurs. Therefore, for practical purposes, animals 

 lack a gametophyte generation. By analogy, one 

 might say that almost all adult animals are compara- 

 ble to a plant sporophyte. 



In our classification of the animal kingdom, we will 

 consider first two groups that are characterized by 

 cellular organization (Mesozoa and Porifera) and two 

 phyla whose differentiation lies in tissue organization 

 (Coelenterata and Ctenophera). All of the remaining 

 phyla possess organs grouped into organ systems of 



increasing complexity. Before reading the following 

 detailed diagnosis of animal phyla, you would do well 

 to review the general discussion of classification in 

 Chapter 7. 



CELLULAR ORGANIZATION: MESOZOA 

 AND PORIFERA 



The cellular animals encompass two subkingdoms. 

 The Subkingdom Mesozoa includes a single phylum, 

 also called Mesozoa, and the Subkingdom Parazoa 

 likewise includes a single phylum, the Porifera. 



MESOZOA 



Diagnosis: cellular, organization; symmetry radial 

 or bilateral; minute, up to about 0.4 inch long; cellu- 

 lar organization consists of an outer layer of cells or of 

 nuclei not separated into individual cells and an inner 

 layer of one or more reproductive cells; the two cell 

 layers not comparable to ectoderm and endoderm; 

 life history complicated, with an alternation of gen- 

 erations; internal parasites; two orders, one parasitic 

 in the kidneys of squids and octopuses, the other in 

 the internal spaces and tissues of various invertebrates 

 (flatworms, ribbon worms, brittle stars, segmented 

 worms, and clams) (Figure 12.1). 



192 



