NUDIBRANCHIATA. 545 



the length being 0'65 cm. and the breadth 0-55. Though the back is arched, the height 

 is small compared to the length and breadth, and the general appearance is flat. The colour 

 is a uniform yellowish brown. The back is perfectly smooth and the mantle edge ample 

 all round. The foot (fig. 2) is however not small : in the preserved specimen its edges 

 are turned inwards, but when extended it must be of a considerable breadth. Its anterior 

 margin is not grooved. The visceral mass is much narrower than either the mantle or the 

 foot. The rhinophores (fig. 3) are yellowish and perfoliate : they appear to be retractile 

 into simple holes without any rim, but in the preserved specimen both were exserted. 

 Above the foot and clearly divided from it is a sort of snout without any trace of tentacles. 

 Considering the size of the animal the buccal mass is large and strong. The edge of the 

 powerful mandibles (figs. 4 and 5) is denticulate, and at the base of the denticulations 

 is a row of blunt knobs. The radula (fig. 6) was somewhat injured in extraction but 

 appeared to be short and to contain about 13 teeth in each row. The character of the 

 teeth was, however, clear. The central tooth consists of a basal plate with a strong cusp: 

 the laterals of a basal portion and a hook: towards the ends of the row the teeth grow 

 more slender and elongate and the hook smaller. Owing to the bad preservation of the 

 interior and the small size of the animal it was not possible to satisfactorily determine the 

 character of the digestive tract and other organs, but it was quite plain that the anal 

 papilla was under the mantle and latero-posterior (fig. 2). A careful examination failed to 

 discover a blood-gland, but its absence cannot be regarded as certain. In the central nervous 

 system all that could be clearly made out was that the optic nerve was long and the eyes, 

 which were large and black, situated unusually deep. At the right anterior comer of the 

 foot was situated the large genital papilla with two distinct orifices (fig. 2). It was un- 

 fortunately impossible to ascertain the character of the internal genital organs, but the 

 hermaphrodite gland appeared to be large and to cover the liver. There were no signs 

 whatever of internal or external gills. 



Fam. Hexabranchidae. 



The femily as at present known is composed of the single genus Hexubranchus, with 

 the following characters. Large animals of a flattish shape and wth ample and undulated 

 pallial margin. Back soft and smooth. Foot broad. Tentacles foliaceous. Rhinophores 

 perfoliate, retractile. Labial armature very strong. Radula composed of numerous simply 

 hamate laterals, without a central tooth. Branchial apparatus consisting of 6 — 8 arborescent 

 tufts, separate fi-om one another, not retractile into a common cavity, but each capable of 

 contracting into a small hollow at the base, which does not however amount to a permanent 

 pocket and does not hide the branchia. Anal papilla within the circle of branchial tufts, 

 central or subcentral. Reproductive organs unarmed. Verge exceedingly long. Nervous 

 system much concentrated, so that it is difficult to distinguish the ganglia. Blood-gland 

 single but large. Near the renal pore there is another small orifice of unknown function. 



The Hexabranchidae differ remarkably from most Dorids in then- habits. They are large, 

 active animals, capable of swimming as well as of crawling. I have generally found them 

 in pools left by the high tide, but think that on these occasions they are merely stranded 

 and naturally prefer deep water. I have also found them swimming on the surface about 

 a quarter of a mile fi-om the coast. Their coloration is generally red and exceedingly 

 conspicuous, in no way resembling their surroundings, but, when at rest, they fold the sides of 



G. II. 70 



