MOLLUSCA 



107 



the egg i.s entirely unknown, that of Chiton only par- 

 tially. Impregnation is effected when the. eggs have lieeii 

 discharged ami are lying beneath the mantle-skirt. A 

 trochosphere larva is developed from the I >i Ma- tula of 

 Chiton (Loven). 



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

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

 menia, and Cluetoderma have hitherto been dredged from 

 considerable depths (100 fathoms and upwards) in the 

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



Sub-class 2. GASTEOPODA ANISOPLEURA. 



C/tar(U'ters. 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 nephridiar 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 



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

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

 condition, , quarter-rotation ; C, complete semi-rotation (the limit) ; rtn,anus; 

 In, r?(, primarily left nephridium and primarily right nephridium ; li'ij, primarily 

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

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

 pig, pleural ganglion ; ptdg, pedal ganglion ; abg, abdominal ganglion ; bucc, 

 buccal mass ; IK, wooden arc representing the base-line of the wall of the 

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

 nerve loop) to W. 



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 7 ' 

 into which the whole animal can be with- 

 drawn, the entrance being often cl" d 

 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 Li- 

 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 

 blood, nor is any ex- 

 pelled during these 

 changes of form. 

 A large mucous 

 gland with a med- 

 ian pore is usually Fl f 

 developed on the the Streptoneurous "con- 



06.1 



ventral surface of 

 the foot, compar- 

 able to the similar 



dition. B, buccal (sub. 

 a-sophageal) ganglion ; t\ 

 cerebral ganglion ; Co, 



Sleural ganglion ; P, pe- 

 al ganglion with otocyst 



attached; p, pedal nerve; 



A, - abdominal ganglion 

 thelong'lo'n'p'cdEutliy- LipOCephala, and in at the extremity of the 

 neurons condition. The / p,, twisted visceral "loop" ; 



untwisted visceral loop some Cases (e.fj. , cy - sp, supra-intestinal visce- 

 is lightly shaded, cc, m la fio- 37 BHhis ral ganglion on the course 

 cerebral ganglion ; pi, . '. ' "'>."' , of the right visceral cord ; 



pleural ganglion : pe, has been mistaken si>,sub-mtestmalganglion 

 pedal ganglion ; a!-, sp, fo - wate _ .,,. on the course of the left 



all.,minal ganglion, 1O1 a "aierpore. visceral cord. (FromGe- 



which represents also The leiotropic genbaur, after Jhenng.) 



gangiion pr of "strepto- torsion of the visceral dome has had 

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



nerve to the osphra- . . , , . , -, . i 



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

 iettered n so^uied I ''Re" ^"S^' as *^ e 1P formed by the two 



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



giiaare omitted! 1 (A?ter caught, as it were, in the twist, we are 



Spengei.) a bi e t distinguish one branch or line of 



descent with straight visceral nerves the EUTHYNEDRA 



Fio. 22. Nervous system of the Pond-Snail, LimnKns stagnalis, as a type of 

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

 its three ganglia i.s lightly-shaded, cf, cerebral ganglion ; pe, pedal ganglion ; 

 j'l, 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 olfactmy ganvlimi and ospnradiom o. In 

 Planorbis and in Auricula (Pulmonata, allied to Limn;cus) the olfactory organ 

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

 (After Spengei.) 



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



