8 THE VOYAGE OF H.M.S. CHALLENGER. 



The Marseniadas possess a protrusible rostrum. The external mouth, situated either 

 at a short distance from the anterior border and then transverse, or further forward and 

 then round, leads into the rostral cavity — the retracted rostrum. When the retraction is 

 complete, the internal mouth, the entrance to the bulbus pharyngeus, is seen on the floor of 

 the cavity as a narrow aperture. When the retraction is only half complete, the rostrum 

 appears within the rostral cavity as a short truncated body, with the interior mouth at its 

 apex and sometimes visible at the external opening. When protrusion is complete, the 

 rostrum is seen as a cylindrico-conical prolongation of some length with the internal 

 mouth at its apex, and with the bulbus pharyngeus therefore within. The internal mouth 

 is surrounded by a flat, narrow lip, and is lined internally on each side by a rather flat, 

 horny mandibular plate. These are either prominent, strongly developed, almost homo- 

 geneous structures, with the anterior border specially thickened {Chelyonotus, Marsenia, 

 Marseniella) ; or they are somewhat narrow plates of a slighter nature, composed of rods 

 arranged in indistinctly separable rows {Marseniopsis) ; or, further, they may resemble 

 the latter in some respects, but with a stronger development, and consist of regular, 

 obliquely transverse rows of component elements with a denticulated anterior edge 

 {Marsenina), and occasionally with a specially strong compound prong in addition 

 (Onchidiopsis). The bulbus pharyngeus is well developed, especially in the Chelyonoti; 

 the anterior portion, with the mandibular plates, is bent slightly downwards ; at each 

 side of the posterior end there is a peculiar " cartilaginous patch," formed from the 

 posterior extremity of the tongue-cartilage, and like it serving for the insertion of muscles. 

 From the middle of the posterior end extends the somewhat long radula sheath, bent 

 usually first down and then up, and often with a slight spiral twisting. The short broad 

 tongue 1 bears very varied armature in the different members of thq group. Usually a 

 strong lateral plate lies on each side of the middle one ; in some genera {Marseniopsis, 

 Marsenina, Onchidiopsis) two external hooks are also present. The number of rows of 

 teeth in the Marseniado3 is, as in the related families, by no means small, amounting in 

 all, including both those on the tongue and those in the sheath, to between 42 and 92. 

 They are most numerous in the Onchidiopsides (as many as 92), and Marseniopsides (as 

 many as 72). The median plates have the posterior margin truncated, or deeply cleft 

 {Marsenia, CJielyonotus, Marseniella) ; the anterior edge is bent upwards, and exhibits 

 small denticles at each side of the strong point. The strong lateral tooth-plates usually 

 have the broad dorsal portion deeply cleft, or only sujierficially grooved {Marsenina, 

 Onchidiopsis). When the tongue is shortened, the point of the body of each of these teeth 

 is pushed into the groove of the plate behind. The strong hook of these plates is toothed 

 on both edges, more finely on the superior (posterior). The two external hook-like plates 

 are but slightly developed, the hooks are not denticulated, or exhibit only a few fine teeth. 



There are apparently no proper salivary glands. 



1 Delle Chiaje seems to have observed the lingual cavity in his Helix neritoidea. 



