M O L L U S C A 



107 



the egg is entirely unknown, that of Chiton only par- 

 tially. Impregnation is effected when the eggs have been 

 discharged and are lying beneath the mantle-skirt. A 

 trochosphere larva is developed from the Diblastula of 

 Chiton (Loven). 



The Chitons are found in the littoral zone in all parts of 

 the world, and are exclusively marine. Neomenia, Proneo- 

 menia, and Chaetodenna have hitherto been dredged from 

 considerable depths (100 fathoms and upwards) in the 

 North Sea, Proneomenia also in the Mediterranean (Marion). 



Sub-class 2. GASTROPODA ANISOPLEURA. 



Characters. Gastropoda in which, whilst the head and 

 foot retain the bilateral symmetry of the archi-Mollusca, 

 the visceral dome, including the mantle-flap dependent from 

 it, and the region on which are placed the ctenidia, anus, 

 generative and nephridial apertures, have been subjected 

 to a ROTATION tending to bring the anus from its posterior 

 median position, by a movement along the right side, 

 forwards to a position above the right side of the animal's 

 neck, or even to the middle line above the neck. This 

 torsion is connected mechanically with the excessive vertical 

 growth of the visceral hump and the development upon 

 its surface of a heavy shell. The SHELL is not a plate en- 

 closed in a shell-sac, but the primitive shell-sac appears 

 and disappears in the course of embryonic development, and 

 a relatively large nautiloid shell (with rare exceptions) 

 develops over the whole surface of the visceral hump and 

 mantle-skirt. Whilst such a shell might retain its median 

 position in a swimming animal, it and the visceral hump 

 necessarily fall to one side in a creeping animal which 

 carries them uppermost. 



The shell and visceral hump in the Anisopleura incline 



cer -y-. 



FIG. 19. Sketch of a model designed so as to show the effect of torsion or rotation 

 of the visceral hump in Streptoneurous Gastropoda; A, unrelated ancestral 

 condition . B, quarter-rotation ; C, complete semi-rotation (the limit) ; ax, anna ; 

 to, rn, primarily left nephridium and primarily right nephridium ; Irg, primarily 

 left (subsequently the sub-intestinal) visceral ganglion ; riy, primarily right 

 (subsequently the sub-intestinal) visceral ganglion ; eery, cerebral ganglion ; 

 pig, pleural ganglion ; ptdg, pedal ganglion ; aby, abdominal ganglion ; biux, 

 bnccal mass; W, wooden arc representing the base-line of the wall of the 

 visceral hump ; x, if, pins fastening the elastic cord (representing the visceral 

 nerve loop) to IT. 



normally to the right side of the animal. As mechanical 

 results, there arise a one-sided pressure and a one-sided 

 strain, together with a one-sided development of the 

 muscular masses which are related to the shell and foot. 

 Both the TORSION THROUGH A SEMICIRCLE of the base of the 

 visceral dome and the continued leiotropic spiral growth 

 of the visceral dome itself, which is very usual in the 

 Anisopleura, appear to be traceable to these mechanical 

 conditions. ATROPHY of the representatives on one side 

 of the body of paired organs is very usual. Those placed 

 primitively on the left side of the rectum, which in virtue 

 of the torsion becomes the right side, are the set which suffer 

 (see fig. 19). Some Anisopleura, after having thus acquired 

 a strongly-marked inequilateral character in regard to such 

 organs as the ctenidia, nephridia, genital ducts, heart,, and 

 rectum, appear by further change of conditions of growth to 

 have acquired a superficial bilateral symmetry, the second- 



ary nature of which is revealed by anatomical examination 



(Opisthobranchia, Xatantia). 



In all groups of Anisopleura examples are numerous in 



which the shell is greatly developed, forming a " house " 

 into which the whole animal can be with- 

 drawn, the entrance being often closed 

 by a second shelly piece carried upon 

 the foot (the operculum). The power of 

 rapidly extending and of again contract- 

 ing large regions of the body to an 

 enormous degree is 

 usual, as in the T.i- 

 pocephalous Mol- 

 lusca. In spite of 

 the theories which 

 have been held on 

 this matter, it ap- 

 pears highly prob- 

 able that no fluid 

 from without is in- 

 troduced into the 

 *P /! blood, nor is any ex- 



pelled during these 

 -o changes of form. 

 A large mucous 

 gland with a med- 

 ian pore is usually 



developed On the *" Streptoneurous ~eon- 

 . t dition. B, buccal (sub- 



ventral surface of 



oesophageal) ganglion ; C, 

 cerebral ganglion ; Co, 

 pleural ganglion ; P, pe- 

 dal ganglion with otocyst 

 attached ; j>, pedal nerve; 

 A, abdominal ganglion 

 thelong-looiidEuthy- LipOCephala, and in * *e extremity of the 



nnn ~*n.,~,..*i:*4 n TV~ r r I Mini mlm 'i 1 "lonn" 



the foot, compar- 

 able to the similar 



neurons condition. The 



twisted visceral "loop" 



u*ricuitu K-"K""" >tj i i . i 



pleural ganglion pe, has been mistaken *o,sub-intestinalganglion 



SSfiPSAa for a water-pore. 3SS $&< 



which represents also The leiotropic genbaur, after Jhering.) 



gangiion P f "strep^ torsion of the visceral dome has had 

 neura and gives off the iggg deep -seated effect in one series of 



nerve to the osphra- , . , F "V . 



dium (olfactory organ) Anisopleura than in another. Accord- 

 leSr^s^died^'^ ^g 1 ?' ^ ^ lo P formed by the two 



nital" ganglion. The VISCERAL NERVES (fig. 19) is Or is not 

 buccal nerves and tsan- i * ^i . 



giia are omitted. (After caught, as it were, in the twist, we are 



SpengeL) a y e ^o Distinguish one branch or line of 



descent with straight visceral nerves the EDTHYSEURA 



FIG. 22. Xervous system of the Pond-Snail, Limneeus stagntil\s, as a type of 

 the short-looped Euthynenrons condition. The short visceral "loop" with 

 its three ganglia is lightly-shaded, ce, cerebral ganglion ; pt, pedal ganglion ; 

 ft, pleural ganglion ; ab, abdominal ganglion ; sp, visceral ganglion of the 

 left side ; opposite to it is the visceral ganglion of the right side, which 

 gives off the long nerve to the olfactory ganglion and osphradinm o. In 

 Planorbis and in Auricula (Pnlmonata, allied to Limmeus) the olfactory organ 

 is on the left side and receives its nerve from the left visceral ganglion. 

 (After Bpengel.) 



(fig. 20) from a second branch with the visceral nerves 



