GENERAL ZOOLOGY 



The tissues of acanthocephalans are largely syncytial, and as in rotifers and 

 nematodes the number of nuclei represented in each organ of the body is 

 relatively constant. Thus, for example, although a protonephridium may 

 consist of hundreds of flame bulbs, the entire organ contains only three nuclei. 



The taxonomic position of the acanthocephalans, and their possible af- 

 finities with other groups, are matters still subject to discussion. In many of 

 their features they seem to show relationships to various members of the 

 phylum Aschelminthes; in others they are somewhat similar to the class 

 Cestoda of the phylum Platyhelminthes. Because they are pseudocoelomate, 

 we have placed them near the aschelminths. It is clear that the Acantho- 

 cephala have behind them an extremely long history of endoparasitism, and 

 that adaptations to this parasitic way of life have been so extensive as to mask 

 their true relationships. 



The Phylum Entoprocta. The members of this small phylum were 

 originally confused with coelenterate polyps and later included with the 

 Ectoprocta as a class of Bryozoa, or moss animalcules. They possess certain 

 well-defined characteristics which make it logical to consider them as a 

 separate phylum, allied to the aschelminths and acanthocephalans. The 

 phylum contains a total of about 60 species; all are marine except members 

 of the genus Urnatella, which inhabit fresh water. With some exceptions, 

 entoprocts are colonial forms, the colony comprising a horizontally extended 

 stolon from which the individual members arise at irregular intervals. Each 

 individual consists of an upright stalk with a cup-shaped calyx at its tip; 

 the organs are contained within the calyx. The upper rim of the calyx bears 

 a ring of ciliated tentacles, which enclose a space termed the vestibule. At 

 one side of this upper surface, within the circlet of tentacles, lies the mouth; 

 the anus also lies within the vestibule, at the side opposite the mouth. The 

 mouth and anus are connected by a U-shaped digestive tract. The organs of 

 a typical entoproct are shown in Figure 12.8. 



Surrounding the digestive tract is a pseudocoelic space largely filled with 

 mesenchyme and containing the reproductive organs, a central ganglion, and a 

 symmetrical pair of protonephridia. Reproduction involves fertilization of the 

 eggs within the oviducts; the zygotes then emerge and attach to the floor of 

 the vestibule, where early embryonic stages are passed. Eventually, free- 

 swimming larvae are produced, which after a short period of free life attach 

 to the substratum and undergo metamorphosis to adulthood. Colony forma- 

 tion follows the growth of a stolon from the base of the stalk of such a 

 solitary individual. 



Like rotifers, the ciliated larvae of entoprocts have been compared with the 

 trochophores of mollusks and annelids (pp. 375, 401), although the simi- 

 larities seem to be highly superficial and may be accounted for on the basis 

 of convergent evolution in response to similar environmental conditions. 

 Probably of greater significance is the marked resemblance between certain 

 entoproct larvae and certain types of rotifers. Since the entoprocts are 

 of the same grade of organization as the rotifers and are like them pseudo- 



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