ZYGOBRANCHIA.] MOLLUSCA 645 twisted into a figure-of-eight the STREPTONEURA (fig. 21). Probably the Euthyneura and the Streptoneura have de veloped independently from the ancestral bilaterally sym metrical Gastropods. The escape of the visceral nerve-loop from the torsion depends on its having acquired a somewhat deeper position and shorter extent, previously to the com mencement of the phenomenon of torsion, in the ancestors of the Euthyneura than in those of the Streptoneura. The junction of the two halves of the visceral loop in the Euthyneura is below the anus, and the loop is therefore not caught by the intestine. In the Streptoneura the junction is (as in the Isopleura) above the anus. Branch a. STREPTONEURA (Spengel, 1881). Characters. Gastropoda Anisopleura in which the visceral " loop " (the conterminous visceral nerves) em braces the intestine and therefore shares in the torsion of the visceral hump, the right cord crossing above the left so as to form a figure-of-eight (see fig. 19). The Streptoneura comprise two orders the Zygo branchia and the Azygobranchia. Order 1. Zygobranchia. Characters. Streptoneura in which, whilst the visceral torsion is very complete so as to bring the anus into the middle line anteriorly or nearly so, the atrophy of the primitively left-side organs is not carried out. The right and left ctenidia, which have now become left and right respectively, are of equal size, and are placed symmetrically on either side of the neck in the pallial space. Related to them is a simple pair of osphradial patches. Both right FIG. 23. Ilaliotis tuberculatu. d, foot ; i, tentacular processes of the mantle. (From Owen, after Cuvier.) and left nephridia are present, the actual right one being much larger than the left. Two auricles may be present right and left of a median ventricle (Haliotis), or only one (Patella). The Zygobranchia are further very definitely characterized by the archaic character of absence of special genital ducts. The generative products escape by the larger nephridium. The sexes are distinct, and there is no copulatory or other accessory generative apparatus. The teeth of the lingual ribbon are highly differentiated (Rhipidoglossate). The visceral dome lies close upon the oval sucker-like foot, and is coextensive with its prolonga tion in the aboral direction. The Zygobranchia comprise three families, arranged in two sub orders. Sub-order 1. Ctcnidiobranchia. Character. Large paired ctenidia acting as gills. Family 1. Haliotidse. Genera : Haliotis (Ear-Shell, Ormer in Guernsey) ; mostly tropical ; Tcinotis. Family 2. Fissurcllidae. Genera: FissureUa (Key-hole Limpet) (figs. 24, 36), Emarginula, Parmophorus (fig. 25) ; mostly tropical. Sub-order 2. Phyllidiobranchia. Characters. Ctenidia reduced to wart-like papillte; special sub- lias been removed, whilst the ante rior area of the mantle-skirt has been longitudinally slit and its sides reflected. , cephalic tentacle ; 6, foot ; d, left (archaic right) gill- plume ; e, reflected mantle-flap ; fl, the fissure or hole in the mantle-flap traversed by the longitudinal inci sion ; /, right (archaic left) nephri- dium s aperture ; g, anus ; h, left pallial lamella, similar to those of the Opisthobranch Pleuro- phyllidia, perform the function of gills. Family 3. Patellidie. Genera : Patella (Limpet, figs. 26, &c.), Nacclla (Bonnet-Limpet), Lottia. Further Remarks on Z ygobranchia. The Common Limpet is a specially interesting and abundant example of the remarkable order Zygobranchia. A complete and accurate account of its anatomy has yet to be written. Here we have only space for a brief outline. The foot of the Limpet is a nearly circular disc of muscular tissue ; in front, projecting from and raised above it, are the head and neck (figs. 26, 30). The visceral hump forms a low conical dome above the sub- circular foot, and standing out all round the base of this dome so as to completely overlap the /* head and foot, is the circular " mantle-skirt. The depth of free mantle-skirt is greatest in front, where the head and neck are covered in by it. Upon the surface of the visceral dome, and extending to the edge of the free mantle-skirt, is the conical shell. When . . ,. . , . /i , FIG. 24. Dorsal aspect of a specimen the Shell IS taken away (best of Fissurella from which the shell effected by immersion in hot water) the surface of the vis ceral dome is found to be covered by a black -coloured epithelium, which may be re moved, enabling the observer to note the position of Some (archaic right) aperture of nephri- i 11 ,1 dium ; p. snout. (Original.) organs lying below the trans parent integument (fig. 27). The muscular columns (c) attaching the foot to the shell form a ring incomplete in front, external to which is the free mantle -skirt. The limits of the large area formed by the flap over the head and neck (ecr) can be traced, and we note the anal papilla show ing through and opening on the right shoulder, so to speak, of the animal into the large anterior region of the sub-pallial space. Close to this the small renal organ (? , mediad) and the larger renal organ (, to the right and posteriorly) are seen, also the pericardium (I) and a coil of the in testine (inf) embedded in the com pact liver. On cutting away the anterior part of the mantle-skirt so as to expose the sub-pallial chamber in the region of the neck, we find the right and left renal papilla? (discovered by Lan- Fio. 25. Pannophorns, seen i /OTX 1 Of"7 ^ ^tli^r oi^ from the pedal surface, o, kester (27) in 1867) on either side mouth; T, cephalic ten- h e an al papilla (fig. 28), but 11O gills, tacle ; fir, one of the two _, -i / i j symmetrical gills placed on If a similar examination be made the neck. (Original.) Q f ^ a uj e( j g enus Fissurella (fig. 24, d), we find right and left of the two renal apertures a right and left gill-plume or ctenidium, which by their presence here and in Haliotis furnish the distinctive char acter to which the name Zygobranchia refers. In Patella no such plumes exist, but right and left of the neck are seen a pair of minute oblong yellow bodies (fig. 28, d), which were originally described by Lankester as orifices
possibly connected with the evacuation of the generative