ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 953 



IV. The last portion of the paper contains a description of an 

 aberrant type of Heteropod, which the author would refer to the 

 Carinaria group. 



Symmetry of Gasteropoda.* — Prof. 0. Biitschli proposes an 

 improvement on Spengel's theory of the derivation of the chiastoneural 

 arrangement in Gasteropods. Like Spengel, he derives the modern 

 forms from a primitive Placophore-like type. In such a form there 

 would be of course two cerebral and two sub-cesophageal ganglia. 

 Besides these in the Chitons there are the two branchiovisceral strands 

 which run along the whole body, and unito posteriorly above the 

 intestine. These Spengel compares with the visceral commissures of 

 Gasteropods which unite the pleuropedals with the so-called abdominal 

 ganglion. The latter gives off nerves to kidney, reproductive organs, 

 heart, &c, and lies ventral to the intestine, like the visceral ganglion 

 of Lamellibranchs. The visceral commissure of Gasteropods is thus 

 ventral, while the branchial visceral strands of Chitons unite dor sally. 

 Spengel acknowledged, but did not get rid of the difficulty ; Biitschli 

 gets over it by denying the postulated homology of the i nerves. In 

 the Placophora tho nerves in question are essentially pallial, and are 

 not homologous with the visceral commissures which do not supply 

 the mantle, but with two distinct pallials which, sometimes at least, 

 meet dorsally. In Chitons the visceral commissure is thus not repre- 

 sented by well-differentiated or distinct branches, but only hinted at 

 by the stomachic nerves, &c. Gradually, however, it may bo supj)osed 

 that visceral commissure and mantle-nerve were distinctly separated. 



From such a primitive form, then, Spengel supposed that the 

 Prosobranchiate chiastoneural arrangement resulted by tho whola 

 complex of organs to right and left and in front of the anus, rotating 

 for 18CT round the latter. Biitschli allows that the rotation has 

 occurred, but differs from Spengel as to its mode. He objects to 

 Spengel's account since it seems to him to overlook the fact that the 

 anus and branchia3 must retain their characteristic position on the 

 pallial groove. The asymmetry of Prosobranchiata, &c, concerns not 

 only the nervous system and the organs generally, but also the 

 intestine and the anus. The anus has been shunted forward, in the 

 pallial groove, on the right side. The formation of a shell implied 

 the more anterior position of the anus, and it is this intestinal 

 asymmetry which has conditioned that of the other organs. At a 

 certain stage of development, when the anus is still at the posterior 

 end, a narrow dorso-ventral zone on the right side between anus and 

 mouth ceases to grow, the corresponding left region continues, and 

 the longer the disproportionate growth continues,* the more is the 

 anus shunted towards tho head. The zone of suppressed growth 

 extends a little beyond the anus to the left side, and. thus the left gill 

 follows the anus. This disproportion is confined to an annular zone 

 of the pallial groove ; foot, mantle, &c, continue growing equally, 

 and an external symmetry is preserved. 



The consequences of this disproportionate growth on tho intestine 



* Morphol. Jahrb., xii. (1S86) pp. 202-22 (2 pis.). 



