Homalozoon vermiculare (Stokes): I. Morphology and Reproduction*

Abstract

SUMMARY. Homalozoon vermiculare (Stokes) is a vermiform holotrichous ciliate of the family Spathidiidae. Its functional ventral side bears 12 longitudinal rows of short cilia. The functionally dorsal side bears a prominent longitudinal ridge, is devoid of cilia, and has very fine pellicular striations. The nuclear apparatus consists of a long folded moniliform macronucleus and several small spherical micronuclei. The contractile vacuoles are variable in number and form a longitudinal row along the functionally right side of the organism.The organism varies from 150 μ to 1500 μ in length and 35 μ to 45 μ in width in its relaxed state. It is highly contractile and its movement resembles that of a turbellarian. The body can be divided into three regions: neck, trunk, and caudal region. The neck region has a cytostome at the anterior end surrounded by a pharyngeal basket and a pair of conspicuous parapharyngeal granular masses at the posterior end. The trunk region is very granular and contains the contractile vacuoles and nuclear apparatus. The caudal region is hyaline and often contains trichite bundles. Free floating trichites are found throughout the organism.Two types of reproduction have been observed. The first type is transverse fission during which the macronucleus separates into two lengths, a new pharyngeal basket forms posterior to the division constriction, and parapharyngeal granular masses develop posterior to the new pharyngeal basket. In the second type of division the cytoplasm condenses in the anterior end of the animal leaving the posterior end very flaccid. The macronucleus then undergoes extensive reorganization followed by the development of a daughter cell from the flaccid posterior region. A new flaccid posterior region grows out of the anterior mass after separation of the first daughter cell and the division process is repeated several times until the remaining portion of the anterior mass disintegrates.