RELATING TO THE ASYMMETRY OF THE GASTROPODA. 145 



are displaced anteriorly to the right, but at the same time they 

 retain their original position with relation to the anterior end, because 

 the region lying between them and the anterior end on the right side 

 • lues not grow. These phenomena have been described by various 

 zoologists who have treated of the ontogeny of the Gastropoda (P. 

 Sarasin, Fol, Bobretzky, etc.).* Spengel (No. 122), also, has 

 made them the subject of detailed consideration in adult animals, 

 and more recently Butschli especially has given a careful descrip- 

 tion of them (No. 19). Lang has recently made a further attempt to 

 explain them from a phylogenetic point of view (No. 61). 



Ontogenetically as well as phylogenetically, the asymmetry rests 

 in any case upon the greater growth of one side, usually the left, 

 and the consequent shifting of the left posterior part of the body 

 to the right and of the whole posterior region anteriorly. In this 

 process we start with a very simple, Chiton-like Mollusc, whose dorsal 

 surface with its investing shell is only slightly arched. The foot 

 projects only a little way beyond the visceral sac. The anus lies at 

 the posterior end, the paired apertures of the nephridia and the gills 

 lying near and symmetrically to it (Fig. 60 ^4). The mantle-cavity 

 also, to which these organs belong, is found at the posterior end. 

 The way in which the shifting forward to the right of this posterior 

 complex of organs (pallial complex) may be imagined to have taken 

 place may best be seen from the diagrams given in Fig. 60 (Butschli 

 and Lang). The asymmetry which is brought about by the shifting 

 of the pallial complex to a position near the anterior end of the body 

 (D) is found in the Opisthobranchia and Pulmonata ; when the 

 pallial complex, in shifting forward, crosses the median line (E), as in 

 the Prosobranchia (including the Heteropoda), the pleuro-visceral 

 commissures become crossed (chiastoneury, streptoneury, Fig. 60 E), 

 a condition not found in the two divisions mentioned above, and 

 indicating a specially high degree of asymmetry. f 



*[It is manifestly impossible in a work of this nature to review all the 

 numerous theories relating to the asymmetry of the Gastropoda. The views 

 adopted by our authors are those of Butschli and Lang, but the reader should 

 consult Simroth's account of the Mollusca in Bronn's Klass. u. Ordnung. d. 

 Thierreichs, Bd. iii. Lief. 22 u. 23, 1896, where an excellent summary of 

 both the earlier and the more recent views, including those of Pelseneer and 

 Plate, will be found. — Ed.] 



t [In Actaeon, a form which, in spite of its peculiarities, must be regarded 

 as most nearly allied to the Opisthobranchs, we find that pleuro-visceral 

 connectives exhibit a streptoneurous condition, and in certain other forms 

 also (I'Jiiliiic, Aplysia, etc., a streptoneurous condition is also found in the 

 Pulmonate genus, Chilina) an indication of this condition is to be seen. The 

 condition met with in these forms is thought to be a highly specialised one, 



L 



