100 SIR 0. ELIOT ON NUDIBRANCHS [May 17, 



BORNELLA. 



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 tentacvilar pi'ocess 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 pan- 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, rouglily triangular, either smooth or denticulate, and a few 

 (9-19) smooth hamate laterals, bent somewhat forward. The 

 innermost are genei-ally veiy 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 thei'e is some irregularity in this respect. The hermaphrodite 

 gland lies on the liver ; the prpeputium 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 intei-nal organs. The number of the cerata and their sub- 

 divisions 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 {B. excepta ; see Bergh's two descriptions), and, 

 when pi-esent, may not extend to all the cerata. The armature of 

 hooks on the prteputium may also be present or absent in the 

 same species {B. diyitata and B. arhorescens ; see Bergh). Nine 

 species are recorded, but B. hermanni Angas, caledonica Crosse, 

 adanisii Gr., semperi Crosse, and hancockaua Kel., will hardly 

 prove valid, for even if they represent specifically distinct forms 

 they are insufficiently characterised. Of the remaining species 

 B. ccdcarata 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. digitata, with a single process behind the rhino- 

 phores, 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. arhorescens 

 are really distinct is discussed below. B. simplex, n. sp., is 

 certainly a separate species, unless it is a monstrosity. 



