Reference : Biol. Bull., 152 : 159—168. (April, 1977) THE FORMATION AND TAXONOMIC STATUS OF THE MICROBASIC Q-MASTIGOPHORE NEMATOCYST OF SEA ANEMONES EDWIN J. CONKLIN, CHARLES H. BIGGER AND RICHARD N. MARISCAL Department of Biological Science, Florida State University, Tallahassee, Florida 32306 Some 27 different types of nematocysts have been described to date from members of the Phylum Cnidaria ( Mariscal, 1974c) . One of the commonest types, the microbasic mastigophore nematocyst, was described by Weill (1934) as having a well defined shaft (enlarged basal portion of thread) less than three times the capsule length. Carlgren ( 1940) later subdivided the microbasic mastigophores into two distinct types, the microbasic b- and microbasic p-mastigo phores, based on the structure of the shaft. The shaft of a discharged microbasic b-mastigophore tapers gradually into the thread, while that of a discharged micro basic p-niastigophore abruptly narrows down to form the thread ( e.g., see Fig. 4—14of Mariscal, 1974c) . The lower portion of the shaft of an undis charged microbasic p-mastigophore has a prominent inverted V-shaped notch, while the microbasic b-mastigophore lacks this feature. Cutress (1955) has described a third type froni sea anemone acontia, the microbasic q-mastigophore, characterized by the possession of a harpoon-like dart which is propelled out of the capsule during discharge. Iwanzoff (1896) was apparently the first to notice this phenomenon, and Weill (1934) has also described it in association with two different nematocysts: the microbasic mastigophore and microbasic amastigophore. The amastigophore is similar to the microbasic mastigophore, but as originally described, lacked a terminal thread beyond the shaft (Weill, 1934). However, as Cutress (1955) has pointed out, a very short fragment of thread may be present, either on the discharged shaft or left behind attached to the inside of the dis charged capsule wall. Iwanzoff (1896), Weill (1934), Hand (1961) and Schmidt (1969) suggest that the dart represents spines from the shaft which have somehow fused together, or otherwise failed to separate, during discharge. These authors do not consider the presence of a dart to be sufficient grounds for the erection of a new category of nematocyst. Cutress (1955), on the other hand, has argued that the dart is an unattached, discrete structure which is consistently found in the capsules of certain nematocysts and characterizes an entirely new type of nematocyst. In order to resolve this controversy and because darts represent one of the most curious features yet described in association with nematocysts, dart-forming nematocysts were ex amined in the present study using both phase contrast and scanning electron micros copy. Specific problems addressed include the relative proportion of darts formed by tentacle nematocysts as opposed to acontial and column neniatocysts, the type (s) of nematocysts capable of forming darts, the number of darts formed by nemato cysts within a particular category, the method of formation of darts and their possible function in the lives of the organisms involved. Based on the information 159