Pleurohrdnchus plumula. 349 



sponge ; the margin thickisb, plain, undulate, free, and suf- 

 ficiently broad to conceal the foot when at rest. Tentacula 

 arising between the cloak and veil, superior, two, cylindrical, 

 short, formed of a membrane folded into a tube slit along one 

 side. Eyes two, small, black; one at the superior base of each 

 tentaculum. Veil above the mouth broad, somewhat triangular, 

 produced at the upper and outer angles, which are folded. 

 Mouth shortly proboscidiform. Space between the cloak and 

 foot smooth, deep. Orifice of the generative organs on the 

 right side, placed very forward, tubercle-like. Branchia arising 

 immediately behind it, single, naked, plume-like, and pectin- 

 ated ; the posterior half free. Foot oval, tapering posteriorly 

 when in a state of extension, and projecting beyond the 

 cloak ; the margins undulate, plain. Shell concealed in the 

 substance of the cloak, dorsal, ovate-oblong, depressed, with 

 a minute spire at one end; brownish, thin, pellucid; strongly 

 wrinkled concentrically, and marked with a slight fossa from 

 the apex to the opposite angle. Length 6 lines, breadth 3 J lin. 



I am unable to describe the internal organisation of this 

 interesting mollusc; but it may be permitted me to direct 

 the student's attention to the wonderful structure of its oral 

 organs, which I could not (although not unfamiliar with ana- 

 logous structures in congenerous species) view without a 

 feeling of indescribable pleasure and amazement: and, to the 

 lovers of the microscope, I am satisfied that no object can 

 afford a more gratifying display. Within the soft parts of the 

 mouth there lie two thin oval plates, one on each side, reti- 

 culated in an inconceivably minute and regular manner, after 

 the fashion of the compound eyes of many insects ; the meshes 

 being diamond-shaped, and set with a small obtuse process at 

 each angle. Between these plates (which, I presume, are a 

 modification of the maxillae or jaws) the tongue is situated : 

 a broad membrane, folded at the sides, and armed with innu- 

 merable little spines or teeth, arranged in close-set transverse 

 series, parting from a longitudinal medial line. The tongue 

 is of a square shape, rounded at the lower end ; to which is 

 appended an inversely heart-shaped piece of similar structure 

 and appearance : the whole fitted to rasp down the vegetable 

 matter on which the animal feeds. 



The cloak is fleshy, but not fibrous ; and, in its composition, 

 includes many small crystaUine spicula of carbonate of lime, 

 which are also to be found in the foot and branchia. These 

 spicula are colourless, short, cylindrical, and rounded at both 

 ends ; and they seem to have no determinate arrangement. 

 I have found similar spicula, but larger and more abundantly, 

 in the tegumentary system of the D5ri5. 



