Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/672

644 MOLLUSCA [ANISOPLEUROUS GASTROPODS. 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 Chaetoderma have hitherto been dredged from considerable depths (100 fathoms and upwards) in the North Sea, Proneomenia also in the Mediterranean (Marion). Sub-class 2. GASTKOPODA 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 an Fio. 19. Diagram to show the effect of torsion or rotation of the visceral hump in Gastropoda, when the visceral nerve commissure passes above the intestine ; A, unrotatcd ancestral condition ; /!, quarter-rotation ; C, com plete semi-rotation (the limit) ; L, left, R, right side of the animal ; an, anus ; In, rn, primarily left nephridium and primarily right nephridium ; Ig, primarily left (subsequently the sub-intestinal) visceral ganglion ; rg, primarily right (subsequently the sub-intestinal) visceral ganglion. Ttie dotted circle indicates the basal area of the visceral hump which undergoes rotation. 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. 1 9). 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 secorid- ary nature of which is revealed by anatomical examination (Opisthobranchia, Natantia). 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 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 S P >rf blood, nor is any ex pelled during these changes of form. A large mucous gland with a med ian pore is usually developed on the ventral surface of the foot, compar able to the similar FlO. 20. NerVOnS SVStem n-lo-nrl anrl liriTO in ofAplysia > asatypeofg lana ana P O1 the long- looped Euthy- Lipocephala, and in neurons condition. The FIG. of the Streptoneurous con dition. B, buccal (sub- ffisophageal) ganglion ; ( , cerebral ganglion ; Co, pleural ganglion ; P, pe dal ganglion with otocyst attached ; p, pedal nerve; A, abdominal ganglion at the extremity of the twisted visceral "loop" ; neurons condition, me / T> v untwisted visceral loop SOHie Cases (e.g., ry- S p, supra-intestinal visce- is lightly shaded, ce, ru l a fin- 37 BHllis ral ganglion on the course cerebral ganglion ; pi, , "*>* & ?) of the right visceral cord : pleural ganglion ; pe, has been mistaken s&,sub-intestinalganghon pedal ganglion ; ab. sp, f wa t e r-DOre "" the , cour f % the J? ft abdominal ganglion, I( r d walel pore. visceral cord. (FromGe- which represents also The leiotropic genbaur, after Jhering.) gangiioTof ^S- torsion of the visceral dome has had neura and gives off the i e ss deep-seated effect in one series of nerve to the osphra- , . , , , A ^ dium (olfactory organ) Anisopleura than in another. Accord- i etT e red n soSd"ge:ingly, as the loop formed by the two nital" ganglion. The VISCERAL NERVES (fig. 19) is Or is not buccal nerves and gan- i , , ,1 , , giia are omitted. (After caught, as it were, in the twist, we are Spengei.) a ^} e t o distinguish one branch or line of descent with straight visceral nerves the EUTHYNEURA Fio. 22. Nervous system of the Pond-Snail, Limnxus stagnalis, as a type of the short-looped Euthyneurous condition. The short visceral " loop " with its three ganglia is lightly-shaded, ce, cerebral ganglion ; pe, pedal ganglion ; pi, 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 osphradium o. In Planorbis and in Auricula (Pulmonata, allied to Limnpeus) 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