I -jn "TERRA NOVA" EXPEDITION. 



ORDER HETERONEMERTINI. 



FAM. BASEODISCIM. 

 Baseodiscus, Diesing, 1850. [= Enpn/i/i, Ilubrecht, 1SS7.] 



4. Baseodiscus antarcticus, sp. n. (IM. I. tigs. 4, 6.) 

 Stations 314 ami 355: McMurdo Sound, I'l'i'-.'SOO fathoms. 



A fairly distinct constriction immediately behind the mouth, when the head is not retracted. 

 Cephalic grooves lateral and vertical. Mouth small and circular. Primary basement-membrane of cutis 

 deep, but loose, and with many radial muscle-fibres. A well-developed layer of gland-cells in connection 

 with the cutis. Bundles of fibres in outer longitudinal muscle-layer of body separated by much gelatinous 

 tissue. Circular muscle-layer thin. Walls of gut not folded. Proboscis slender, and proboscis-sheath 

 thin-walled. 



Two specimens which I refer to this form occur in the collection. 



The larger of the two measures 5 '5 cm. in length, ami lias a maximum thickness 

 of !) mm. The smaller, which is apparently a young female, measures oulv 2 cm. in 

 length and 5 mm. in thickness. 



There is no trace of colour or markings upon either individual. 



EXTERNAL FEATURES. 



In the small specimen the characters of the head (PI. I, fig. 4) can be fairly well made 

 out ; it is marked off from the body by a moderately distinct constriction, immediately 

 behind the mouth. The proboscis-pore (P.P.) is a well-marked vertical slit just below 

 the apex of the head. The shallow cephalic grooves ((jr.) are lateral and vertical, and 

 apparently do not form a complete ring. The mouth (M.) is small and circular, with 

 regularly wrinkled margin. 



In the larger example the head is much retracted, and little of these features can 

 be made out with certainty. 



INTERNAL ANATOMY. 



A small piece was taken from about the middle of the body of the small specimen, 

 and cut into transverse sections (PI. I. fig. G). These reveal the following features: 



The external epithelium of the body (Ep.) consists of tall cells, resting on a 

 secondary basement-membrane (B.M 2 .), succeeded by two thin Livers of muscle-fibres, 

 an outer circular and an inner longitudinal. Beneath the latter is a well-developed 

 layer (Gl.) of large glandular cells. Next comes the thick primary busement- 

 membrane (B.M 1 .), consisting of a rather loose connective tissue, through wliich many 

 bundles of muscle-fibres pass outward radially. 



The outer layer of longitudinal body-muscles (L.M 3 .) comes next in order. The 



