108 



MOLLUSCA 



twisted into a figure-of-eight the STREPTONETJRA (fig. 

 21). Probably the Euthyneura and the Streptoneura 

 have developed independently from the ancestral bilater- 

 ally symmetrical 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 commencement of the phenomenon of torsion, 

 in the ancestors of the Euthyneura than in those of 

 the Streptoneura. In the ancestral Streptoneura the 

 visceral loop was lateral and superficial as in the living 

 Isopleura. 



Branch a. STREPTONEURA (Spengel, 1881). 



Characters. Gastropoda Anisopleura in which the 

 visceral " loop " (the conterminous visceral nerves) was 

 adherent to the body-wall and so shared 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. Zygoforanchia. 



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. Kelated 

 to them is a simple pair of osphradial patches. Both right 



FIG. 23.Haliotis tuberculata. 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 Zygobraiicliia comprise three families, arranged in two sub- 

 orders. 



Sub-order 1. Ctcnidiobranchia. 

 Character. Large paired ctenidia acting as gills. 

 Family 1. Ifaliotidse. 

 Genera : Haliotis (Ear-Shell, Ornicr in Guernsey) ; mostly tropical ; 



'/' inotis. 



Family 2. Fiaxin-rlliiln-. 



Genera: Fissurdla (Key-hole Limpet) (figs. 24, 36), Emaiyiniiln, 

 us (fig. 25) ; mostly tropical. 



Sub-order 2. P/iyllidiobranchia. 

 . Ctenidia reduced to wart-like papilla:; special sub- 



pallial lamella;, similar to those of the Opisthobranch Pleuro- 

 phyllidia, perform the function of gills. 

 Family 3. PatcUidse. 



Genera: Patella (Limpet, figs. 26, &c.), Xacclla (Bonnet-Limpet), 

 Lottia. 



Further Remarks on Zygobranchia. 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 

 the shell is taken away (best 

 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 

 organs lying below the trans- 

 parent integument (fig. 27). 



Fio. 24. Dorsal aspect of a specimen 

 of Fissurella from which the shell 

 has been removed, whilst the ante- 

 rior area of the mantle-skirt has 

 been longitudinally slit and its sides 

 reflected, a, cephalic tentacle ; b, 

 foot ; d, left (archaic right) gill- 

 plume ; c, reflected mantle-flap ; fi, 

 the fissure or hole in the mantle-flap 

 traversed liy the longitudinal inci- 

 sion ; /, right (archaic left) nephri- 

 dium's aperture ; </, anus ; ft, left 

 (archaic right) aperture of uephri- 

 dium ; p, snout. (Original.) 



The muscular columns (c) 



attaching the foot to the shell form a ring incomplete in 

 front, external to which is the free 

 man tie -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 (i, mediad) and the 

 larger renal organ (/, to the right 

 and posteriorly) are seen, also the 

 pericardium (/) and a coil of the in- 

 testine (int) embedded in the com- 

 pact liver. 



On cutting away the anterior part 

 of the mantle-skirt so as to expose 



Vthe sub-pallial chamber in the region 

 / of the neck, we find the right and 

 left renal papilla* (discovered by Lan- 



XffiSSSSSa."tkter-() in 1867) on either side 

 mouth; T, cephalic ten- the anal rial lilla (fig. 28), but no gills. 



tacle; br, one of the two _, . i ., 1 v . '. , 



symmetrical gills placed ou If a similar examination be made 

 the neck. (Original.) of t]le all ; cd genug Fi ssure H a (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 



