TRITONIA. 



A pretty full account of the anatomy of Tritonia, is given in the ' Memoires des Mollusques' 

 of Cuvier ; but, requiring more complete details, we have thought it desirable to re-investigate 

 the entire subject, and have in consequence carefully dissected T. Hombergii, the anatomy of 

 which we purpose giving at considerable length ; Tritonia being the type of our second family. 



The oral opening is placed on the inferior surface of the head, and is guarded by a 

 wrinkled lip, which is divided by a fissure into two lateral lobes. The channel leading from 

 thence into the buccal organ is very short. The buccal organ itself (fig. la, and f. 3) is of 

 vast magnitude, exceeding in this respect that of all other Nudibranchs. It is very muscular, 

 somewhat depressed, of an irregular quadrate form, and provided with powerful corneous 

 jaws, which extend the whole length of the organ, imbedded in the muscles. The jaws 

 (figs. 7, 8) are much elongated, rather narrow, with the anterior extremities arched upwards, 

 and firmly united by an elastic cartilage. The inner margins are thick, forming serrated 

 cutting blades (figs, lb, 8b). 



Three sets of muscles are employed in the movement of the jaws ; one of which (fig. Ad, 

 and fig. 3<?) passes across in front of the cutting blades above, and another {Ad' and 3<?') below, 

 having the oral opening between them ; their extremities are inserted into the external surface 

 of the jaws towards their outer margin. These two sets, acting in unison, will bring the 

 cutting blades together, whilst the elasticity of the hinge, aided by the third set {Ae and 3/) will 

 separate them. This last-named set lies transversely over the hinge, and is attached by its 

 two extremities to the jaws ; thus it will not only assist in closing them, but also in binding 

 them together. A very similar arrangement of muscles for moving the jaws exists in Eolis. 

 The buccal mass and jaws in T. alba and T.plebeia, are like those of T. Hombergii, but in both 

 the former the cutting blades are furnished with a peculiar denticulated belt (fig. 10c, and fig. 

 11$) on the outer surface. It is likely that in these instances the jaws are principally 

 prehensile organs, holding the prey by the aid of the denticulated belt, while the tongue is 

 employed in tearing off piece hj piece, and so transporting it to the oesophagus. A similar 

 belt has been observed in the young of T. Hombergii, but not satisfactorily in the mature animal. 



The tongue of T. Hombergii is very large, filling up the greater part of the buccal cavity, 

 and is so placed that the anterior or upper portion of the dentigerous membrane (fig. Af) is 

 opposed to the oesophagus. This membrane (fig. 5) is formed on the plan of that of Doris 

 tuber culata, being broad and tubular, with the anterior portion {a) expanded somewhat like the 

 mouth of a trumpet. The expanded portion only, which forms, as it were, two lateral lips, is 

 brought to act upon the food ; and the lingual pouch does not extend backwards beyond the 

 buccal mass. The operative part is divided by a fleshy process from that ,which is more 

 distinctly tubular, and within which the spines are developed. 



The dentigerous membrane in T. Hombergii, when removed from the lingual muscles 

 and spread out, is broader than long ; and the teeth or spines (fig. 6) are long, simple, and 

 very numerous. They are arranged in eighty-four transverse rows, each containing four 

 hundred and forty spines, and a central (b) and two lateral plates (c, c). The central plate is 

 rather large and of a quadrate form, with three not very distinct, obtuse denticles. The lateral 

 plates are irregularly oval, each bearing a blunt projection. In T. alba the membrane is 

 considerably longer than broad, and is supplied with twenty-nine rows of about seventy-two 

 spines each, together with a central and two lateral plates. The spines are long, arched, and 

 denticulated : the central plate has three stout denticles ; the lateral plates bear an elevated 



