Diagnosis: A characteristic of living animals is that they attach themselves to the substratum by means of the posterior body end (Marcus 1948). Further, the species may be recognized by (1) its characteristic arrangement of the intestinal branches, (2) the two large testes in front of the pharynx, and (3) its sclerotized penis papilla.
Habitus: Preserved specimens vary from elongate to oval-shaped; front and hind end are broadly rounded. The size of preserved specimens ranges from 0.4 mm in length and 0.25 mm in breadth up to 0.7 x 0.325 mm; maximally stretched specimens measure 1 x 0.22 mm (Marcus 1948). The animals are unpigmented. The eyes are set close together at some distance from the anterior margin.
The pharynx measures about one-sixth to one-third of the body length. The inner circular muscle layer of the pharynx is well developed, with the outer circular muscle layer being as thick as, or even slightly thicker, than the first-mentioned layer. The mouth opening is situated at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends anterior to the eyes and gives off a pair of short preocellar diverticula. Between the eyes and the root of the pharynx a large divertculum arises from either side of the anterior gut trunk. Each diverticulum extends forwards beyond the eyes and gives rise to 4-5 secondary lateral diverticula. Each posterior intestinal ramus gives off about 7 lateral diverticula and also several short, medially directed branches. The posterior intestinal rami have a commissure behind the copulatory apparatus. The intestinal diverticula are usually unbranched but the secondary diverticula arising from the pair of anteriorly directed diverticula, may be forked.
Male Reproductive System
Two large testicular follicles are situated just in front of the pharynx, behind the pair of large, anteriorly directed diverticula which arise from the anterior gut trunk. The testes are situated ventrally but extend dorsally well beyond the midline of the body.
The vasa deferentia are very narrow and only enlarge behind the pharyngeal pocket, shortly before opening into a large seminal vesicle which lies just inside the penis bulb. The ejaculatory duct arises from the posterior surface of this vesicle. At first, the last-mentioned duct is rather broad but beyond the point where it receives the openings of the penis glands, it becomes very narrow. The ejaculatory duct opens at the tip of the penis papilla.
The penis has an oblique, ventro-caudal, orientation. It consists of a shallow bulb and a large, conical papilla. The proximal portion of the papilla is rather broad, but its distal part consists of a large and narrow, sclerotized tip. Penis glands, which lie dorsally and ventrally of the penis bulb, discharge their secretion into the proximal section of the ejaculatory duct.
The musculature of the penis bulb is very weakly developed, but the proximal portion of the penis papilla is provided with a well-developed layer of circular muscle fibres.
Female Reproductive System
The paired ovaries are situated directly behind the brain and in front of the pair of large, anteriorly directed, lateral diverticula that branch off from the anterior intestinal ramus. The ovaries are U-shaped, the open part of the U being oriented towards the ventral body surface. The ventro-anterior portion of the U contains the germ centre.
The atrium communicates with a dorsally located expansion via a narrow, nucleate and ciliated canal. This atrial expansion gives rise to an anteriorly directed, sac-shaped extension on both sides of the atrium. The latter, including its expansion, is lined with nucleate cells without cilia. The oviducts, which run laterally to the ventral nerve cords, meet the ventro-lateral wall of the atrial expansion. Sperm may be present in the expansion and its anterior extensions.
A duct arises from either side of the postero-ventral surface of the atrial expansion; these ducts open ventrally to the exterior. The ducts are lined with a nucleate epithelium which bears well-developed cilia, while sperm may be present in their lumina. The openings of these ducts are situated somewhat posteriorly to the gonopore, i.e. postero-laterally to the neural arch formed by the ventral nerve cords. The gonopore lies at the hind end of the atrium, i.e. ventrally of the duct communicating with the atrial expansion. In most of the specimens examined, the gonopore was closed due to contraction.
Information on the musculature of the female copulatory apparatus could not be obtained. Extensive shell glands open into the posterior portion of the atrium.
The vitellaria are well developed and extend from anterior to the ovaries to posteriorly to the copulatory apparatus.
The eye cups have a large semi-circular lens in front of their openings. I found it impossible to determine the number of retinal cells, but according to Marcus (1948) three of these cells would be present in each pigment cup. The eyes lie on the brain but are not embedded in it.
Nerpa evelinae has been collected from algae, especially Dictyota (Marcus 1948).
Type locality: Guaruja, near Santos, Brasil. The species is known only from the type locality and another locality in the bay of Santos.
Material Examined and Type Material
Uncatalogued slides from Dept. Zoologia Sao Paulo : 1 slide with with 2 stained whole mounts, each with a cocoon in the atrium; 1 slide with 2 stained whole mounts; 1 slide with 3 unstained whole mounts; 1 slide with transverse sections of 2 specimens; 1 slide with transverse sections of 2 specimens; 1 slide with sections of 2 specimens, one sectioned transversally, the other sagittally; 1 slide with sections of 4 specimens, two sectioned sagittally, one transversally and one horizontally.
S.M.N.H.: 3 whole mounts on 1 slide; 1 slide with serial sections of six specimens; 1 slide with serial sections of two specimens.