Four known species of Loxosoma, namely, L. phascolosomatum Vogt, L. crassicauda Salensky, L. singulare Keferstein, and L. claviforme Hincks, and a new species L. obesum are found in the Plymouth region, and are described.
L. phascolosomatum is found on Phascolosoma vulgare, and in addition on two molluscs, Lepton clarkiae and Mysella bidentata from the burrows of Phascolosoma (pellucidum) elongatum from the Salcombe Estuary.
L. crassicauda lives in the tanks in the Laboratory. Its average length is 1.4 mm. Between March 1929 and February 1930 males only were found: no ova were seen.
L. singulare.--Occurs on Aphrodite aculeata; it varies between 0.18 and 0.8 mm. in length. In females carrying embryos the vestibule has two diverticula, one on either side of the rectum.
L. claviforme.--It is considered a valid species, and may be distinguished from L. singulare by: (1) its greater size and length of stalk, (2) greater number of tentacles (commonly twelve), (3) position of the budding zone, and (4) the presence of paired sense-organs. Its average length is about 0.8 mm. It occurs on Hermione hystrix.
A small group of Loxosoma, found on Aphrodite aculeata, were intermediate in form between L. singulare and L. claviforme, and were peculiar in retaining a number of their buds. The sex of such buds in several instances differed from that of the parent.
L. obesum sp. nov. is found on the dorsal surface of Aphrodite aculeata. It may reach a length of 2.4 mm.; average individuals are rather more than 1.0 mm. in length. The lophophore is small, and bears almost invariably eight tentacles. Longitudinal muscles only are present in the stalk, which ends in a small disc of attachment. A foot-gland is present in the bud, and is frequently preserved as a vestige in the adult. The buds are near the lophophore, and may be as many as six on either side. The larva resembles that of L. singulare.
Two main forms may be distinguished, differing in shape of the calyx and development of the stomach.
The ovary may contain six well-developed ova on either side, and the vestibule twenty-six embryos.
With one exception, females greatly exceeded males in number, and it is probable that the male becomes sexually mature at a smaller size than does the female.