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The literature pertaining to the morphology of the various Carcinonemertes species shows that in most respects they exhibit a remarkable uniformity in structure . Humes (1942: 76), however, provided a key to the species (excepting C. errans which has been described subsequently by Wickham, 1978) in which he separated C. coei from the remainder by "Ovaries in two rows on each side of the intestine, an inner and an outer row ; basis twice as large in diameter posteriorly" . In the other forms the ovaries are in only a single row on each side of the body, as in the present species. Carcinonemertes epialti distinguished by its "Posterior proboscis chamber bent dorsally at an angle to the proboscis ; body less than 6 mm long" (Humes, 1942 : 76 ; see also Coe, 1902: fig. 6), quite unlike the situation in the present or other members of the genus. The Japanese species, C. mitsukurii, Humes distinguished as having the "Anal blood vessel commissure dorsal to posterior end of intestine", whereas in both varieties of C. carcinophila the anal vascular connective passes below the alimentary tract . In the Australian specimens the condition is the same as in C. mitsukurii.Wickham (1978)
subsequently distinguished C. errans from C. epialti, the only other species recorded from the Pacific coast of North America, by its numerous small epidermal spots which gave the body an opaque appearance, and by the size of the proboscis armature. Wickham's (1978) incomplete description of C. errans does not afford a detailed comparison with the present species, but the Australian form in life has a transparent epidermis with no trace of the white spots found in C. errans, and both the absolute dimensions of the stylet apparatus and the stylet:basis ratio are different ; Wickham reported that in C. errans the basis is 35 .1 ± 0 .3 micrometers long, 10 micrometers wide posteriorly and tapering to an anterior diameter of 5 micrometers ; the stylet is 11.0 ± 0.2 micrometers long, giving a stylet:basis ratio of 0.314. These values contrast with the dimensions and ratio recorded for the present species. The size of the stylet apparatus in the Australian form also serves to distinguish it from C. mitsukurii; Takakura (1910) noted that in this form the stylet was 8 um long, the elongate oval basis up to 27 micrometers long, whilst Humes' (1942) examples of the same species possessed styles 8.5 micrometers long and bases 20.5-22.5 micrometers long and 5.0-8.0 micrometers wide. Other differences between C. mitsukurii and the present species are that in the former the cephalic vascular loop is located immediately in front of the brain rather than further anterior toward the tip of the head, and the lateral nerve cords meet posteriorly by a subintestinal, rather than a supraintestinal, commissure . We conclude that these differences are sufficient to exclude the Australian form from any of the known species or varieties of Carcinonemertes and it is accordingly placed in a new taxon, C. australiensis sp. nov.