194 



MESOZOA THROUGH ENTOPROCTA: 



Figure 12.1 Two types of mesozoans. Drawn to indicate both ex- 

 ternal and internal structure: Dicyema to the left and Rhopalura to the 

 right. (Modified from various sources.) 



We have already examined the conflicting ideas of 

 biologists on the structure of the Protozoa; the addi- 

 tional fact that scientists do not agree on the phylo- 

 genetic position of the Mesozoa might cause the 

 reader to believe that biologists are an argumentative 

 group. If one is more serious about the various con- 

 troversies in biology, he realizes that they exemplify a 

 dictum of science: nothing should be accepted as fact 

 whenever there is room for disagreement. All possible 

 explanations of any phenomenon must be considered. 



Some zoologists believe that mesozoans are a de- 

 generate group of free-living flatu'orms. The idea that 

 they might be degenerate flatworms stems from their 

 being parasites and their simplified flatworm-like 

 structure. Moreover, parasites tend to become struc- 

 turally simplified, often losing many of their organs. 

 Many parasites are little more than nutrient-absorb- 

 ing, reproductive organs. Other zoologists believe 

 that mesozoans are exactly what they appear to be, a 

 group that lies structurally between the Protozoa and 

 the Eumetazoa — hence their name, "middle ani- 

 mals." This latter interpretation has gained favor be- 

 cause mesozoan surface cells, which have the ability 

 to engulf and digest food, are far removed from the 

 kind of cells found in flatworms. Furthermore, one 

 might say that these animals resemble some ciliate 

 protozoans: both groups have cell-like structure, or 

 cells that ingest and digest food; have cilia through- 

 out much of their life history; and have an internal 

 layer of specialized, reproductive "cells." 



The Mesozoa are also of interest because they have 

 two cell layers that are not comparable to germ layers 

 and have one or more life cycle stages that are un- 

 known. For example, in one order, parasitic in octo- 

 puses and squids, only the sexual cycle is known; an 



asexual cycle, which seemingly must exist, is not 

 known. 



PORIFERA (Sponges) 



Diagnosis: cellular organization; symmetry radial 

 or asymmetrical; occur as thin flat crusts, plant-like 

 branches, and vase-like globular or irregular masses; 

 colors range from gray to drab to brilliant red, 

 orange, yellow, blue, violet, or black; adults attached 

 to rocks or other objects; without mouth or organs of 

 any kind; body permeated with pores, canals, and 

 chambers through which a water current flows; body 

 generally rigid and spiny or velvety to the touch, 

 rarely slimy; marine, except for one fresh- water 

 family. 



This phylum may be confused with the Ectoprocta, 

 Entoprocta, some Chordata (tunicates), and some 

 Algae. Sponges, however, usually have one or more 

 visible openings, depending upon solitary or colonial 

 forms; the other phyla do not. Also, sponges tend to 

 be unique in having readily visible internal cavities. 

 Individuals in a sponge colony mass are rarely dis- 

 cernable, but Ectoprocta, Entoprocta, and the 

 Tunicata usually are, even though individuals in 

 Ectoprocta, Entoprocta, and Tunicata may be mi- 

 nute. The Algae form smooth, usually rock-like and 

 purplish crusts. 



The sponges are many-celled animals, but unlike 

 all other multicellular animals (except the Mesozoa) 

 sponge cells are not combined into tissues. The indi- 

 vidual cells function much like members of a proto- 

 zoan colony, so sponges are organized on a cellular 

 basis. These and other features have caused them to 

 be classified as the Subkingdom Parazoa, "beside 

 animals," implying that they stand beside rather than 

 among the other animals. 



Simple, solitary, adult sponges are vase-like, seden- 

 tary animals (Figure 12.2). They possess, in certain 

 surface cells, pores that lead into a large central 

 cavity which has a single, large, mouth-like opening 

 to the exterior. The central cavity is lined by flagel- 

 late cells with the whip-like structures surrounded 

 by a collar. These cells produce a current into the 

 central cavity through the surface pores and out of the 

 cavity through the large opening. This simple or- 

 ganization is called the ascon type. In the more com- 

 plex organizations the current-producing cells are 

 found not in the central cavity, but in special cham- 



