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Show 100 SIR C. ELIOT ON NUDIBRANCHS [May 17, B o r n e l l a . The members of this genus are slender, elegant animals, having on either side of the back a row of cerata mostly divided into 2-4 branches and bearing gills. On either side of the mouth is a compound tentacular process consisting of a number of conical tubercles set in a sort of rosette. Over the head are a pair of large organs called in the following descriptions for brevity's sake rhinophore-sheaths, but apparently formed by a fusion of the true rhinophore-sheaths with a pair of cerata. The pair of cerata after these organs are called the first pair. The vent is latero-dorsal between the first and second pair of cerata. The buccal mass is not large, but very muscular ; besides the jaws and radula, there is also a labial armature of scales. The radula consists of a median tooth, roughly triangular, either smooth or denticulate, and a few (9-19) smooth hamate laterals, bent somewhat forward. The innermost are generally very small, and the size increases towards the outside of the row. There are two stomachs, of which the second is armed with spines, and two accessory livers, besides the main mass. As a rule ramifications of the liver enter the cerata, but there is some irregularity in this respect. The hermaphrodite gland lies on the liver; the prseputium is smooth or armed with spines. There is considerable difficulty in dividing the genus into species. The colour presents little variety, being in all the known forms whitish yellow, with a red or yellow reticulation on the back. On the other hand, there is some variety in both the external and internal organs. The number of the cerata and their subdivisions appears not to be specifically characteristic, but to increase with age, and is not always the same on the two sides of the body. The ramification of the liver may be present or absent in the same species (/>. exceptci; see Bergh's two descriptions), and, when present, may not extend to all the cerata. The armature of hooks on the prseputium may also be present or absent in the same species (B. digitata and B. arborescens; see Bergh). Nine species are recorded, but B. hermanni Angas, caledonica Crosse, adamsii Gr., semperi Crosse, and hancockana Kel., will hardly prove valid, for even if they represent specifically distinct forms they are insufficiently characterised. Of the remaining species B. calcarata Morch, from the West Indies, is distinguished by having appendiculate rhinophore-sheaths and smooth median teeth. The Indo-Pacific forms fall into two groups-the one represented by B. digitcita, with a single process behind the rhinophores, cerata divided into rather long erect fingers, and median teeth with faint denticulations ; the other by B. excepta, with several processes behind the rhinophores, small fingers on the cerata protecting the external branchiae, and much more distinctly denticulate median teeth. Whether B. digitata and B. arborescens are really distinct is discussed below. B. simplex, n. sp., is certainly a separate species, unless it is a monstrosity. |