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A catenulid flatworm from the northernmost saprobic edge benthos of the spring-fed freshwater coastal pond at Ocean Dunes Apartments in the Atlantic Double Dunes Reserve. The sampling site is located 250 meters from the edge of the Atlantic Ocean. Imaged in Nomarski DIC on Olympus BH2S using SPlan 20x and 40x objectives plus variable phone cropping on Samsung Galaxy S9+. This is quite possibly Catenula confusa, the most common species in the eastern USA. The auricular expansions at the posterior cephalic region are characteristic as is the 2- zooid chain. I show here a chain of two zooids in paratomy markedly compressed by the cvoverslip and together measuring 1.0 mm in length. The second individual is free swimming and not compressed by the coverslip.

Catenulida is an order of turbellarian flatworms in the classical classification, or a class of flatworms in a phylogenetic approach. They are relatively small free-living flatworms, inhabiting freshwater and marine environments. There are about 100 species described worldwide, but the simple anatomy makes species distinction problematic. All catenulids are aquatic, benthic animals. Most of them live in freshwater, being usually very abundant in mires, ponds, streams and moist terrestrial habitats. A small number of species are known to live in the sea.

The anatomy of catenulids is simple and lacks hard parts. The mouth is located anteriorly and connects to a simple pharynx and a simple intestine that forms a ciliated sac. They possess two pairs of nerve cords and often a statocyst, as well as a single protonephridium. The gonads are unpaired. Unusually, the male gonopore opens on the dorsal surface of the animal, above the pharynx, while the female reproductive system lacks any of the usual ducts and related structures found in other flatworms. The sperm is nonmobile and lacks flagella or cilia. Asexual reproduction by paratomy is common in which one zooid develops behind the other forming a chain of offspring (zooids), hence the name, from Latin catenula, small chain.

"The genus Catenula was established in 1832 by Duges for a fresh-water worm which he collected in autumn, 1830. He proposed the name Catenula because of the resemblance of a specimen of several zooids to a small chain; the specific name lemnae was used because he had discovered the animal in a lens of water. Duges' description is somewhat incoherent and is confused by the fact that throughout the paper he becomes involved in comparisons with tapeworms, other flatworms, and annelids. However, his figures are good and, with the description given, he established beyond doubt the fact that he was dealing with the same species we have cultured for seven years.

Leidy (1851) described Anortha gracilis from Philadelphia which so early as 1854 was recognized by Leuckart as a Catenula and carried by him as Catenula gracilis. Catenula gracilts was carried as a species by practically every author who mentioned the genus until Graff (1913) relegated it to synonymy with Catenate lemmae. From Leidy's description it is difficult to see how it was specifically identified at all. Schmarda (1959) added C. bina and C. quarterna to the genus. C. bina lacked a statocyst and C. quarterna had a protonephridial tube down each side of the body. However, the description of C. quarterna leaves no doubt that Schmarda was dealing with an organism of the same general structure as a Catenula. It was suggested by Beauchamp (1919) that such a protonephridial condition would justify removal to another group. Luther (1907) suspected that Schmarda had committed an observational error with reference to the two protonephridial tubes, and Marcus (1945a) considers the inclusion of C. quarterna as a valid species open to question but, nevertheless, includes it in his key for the genus. In view of the fact that this species conforms so completely in other respects to the generic criteria, it seems logical that these criteria be revised to allow the inclusion of this species.

Graff (1882) moved C. lemnae, C. gracilis, C. Irina, and C. quarterna into the genus Stenostomum in the family Microstomidae, but in 1913 he reestablished the genus Catenula in the family Catenulidae with two species —C. Iemnae and C. quarterna. Since 1913 six species have been described. From Europe Beauchamp (1919) described C. sekerai and Reisinger (1924), C. pygmaea. From Virginia, Kepner and Carter (1930) described C. virginia. From Brazil, Marcus (1945a) described C. alitha, C. leuca and C. macrura.

Beauchamp (1919) undertook to separate C. sekarae from C. lemnae on the basis of anatomical features, but particularly on the difference in the histology of the parenchyma. The species with which he was comparing his new species appears not to have been C. lemnae but an undescribed species very widely found in the eastern United States. This species is described in this paper as C. confusa.

Catenula confusa has been collected from ponds and roadside ditches through-out the eastern United States. It is the most common of all species of Catenula collected. Single zooid specimens range from .354 to .5 mm in length; eight zooid specimens, 3.2 mm; extended worms measure from .083 to .091 mm in width. Chains of two to four zooids are of most frequent occurrence. The general form is threadlike; the anterior end is rounded and the posterior end tapers into a blunt point. A deep pre-oral groove separates the cephalic region from the rest of the zooid. Developing fusion planes of the more mature zooids form prominent constrictions at intervals in a chain. The ratio of the length of the cephalic region to the total length of the zooid in a single zooid specimen is about 1:5; in a two zooid specimen the ratio of the anterior zooid's cephalic region to the chain becomes approximately 1:8.

Anteriorly the cephalic region is rounded; posteriorly it is symmetrically expanded into lateral lobes. In reflected light the worm is milky white in appearance. Greenish granules are scattered throughout the parenchyma. The cilia of the general ciliary coat measure about .01 mm in length. In the region of the pre-oral groove the cilia are thicker and longer. The cephalic ganglion conforms to the general shape of the head. It is prolonged anteriorly into a point, but widens considerably between the lateral cephalic lobes. Embedded dorsally in the tissue of the posterior part of the ganglion is the statocyst which measures from .016 to .018 mm in diameter in mature zooids. The statoliths vary in diameter from .004 to .006 mm. The V.shaped mouth is situated ventro-posteriorly to the pre-oral groove. It is bounded by muscular, heavily ciliated lips and opens into a short, muscular, ciliated pharynx. The pharynx opens through a sphincter into the ciliated enteron. In the mid-region of each zooid the enteron is distinctly widened, and as it approaches the posterior end of the zooid it narrows gradually. Irregularly shaped cells fill the pseudocoel. Muscle strands traverse the pseudocoel in all regions of the body, and are particularly prominent at the anterior and posterior ends of the body. In contracted specimens the parenchymatous cells appear to be organized into irregular rolls. The protonephridial system follows the generic pattern. The nephridiopore opens at the posterior end of the body. This species does not culture under laboratory conditions. Specific diagnosis.—Anterior cephalic region bluntly rounded; cephalic auricles at posterior end of cephalic region; cephalic region distinctly less than one-half the length of the body of a mature zooid; 1-8 zooids; statocyst embedded in postero-dorsal aspect of ganglion" (1).

1. The Catenula of the Eastern United States. John W. Nuttycombe. The American Midland Naturalist, Vol. 55, No. 2 (Apr., 1956), pp. 419-433

A digeneid trematode from a snail host (Nassarius) . Causes cercarial dermatitis in humans.. Microscope image shows cercariae being squeezed out of a redia.
The snail's infested viscera are shown in a macro photo as well. The snail was anesthetized before shell removal/dissection.