THE FORMATION OF THE ORGANS THE NERVOUS SYSTEM. 193 



which was observed in various Prosobranchs (SARASIN, WOLFSON, HADDON, 

 MCMURRICH, v. ERLANGER, etc.), in Heteropoda (FoL) and perhaps also in 

 Opisthobranchs (RAY LANKESTER) appears in any case to be the more usual. 

 The pedal ganglia arise laterally or rather on the under surface of 

 the foot, near the otocysts, the position of which has already been 

 described more than once (Fig. 88 B, p). These ganglia at first are 

 not connected with each other nor with any other ganglia. The 

 commissures and connectives * are secondary structures, i.e., they 

 arise only after the detachment of the ganglia from the ectoderm as 

 outgrowths of the ganglia, a point on which the statements of all 

 observers agree. Where, as in the Pulmonates, the ganglia lie close 

 together, the distinct ganglia, in the course of growth become 

 connected at an early period. 



Besides the original commissure connecting the pedal ganglia in the 

 Pulmonates, a second smaller commissure appears lying more posteriorly. 

 Since this second commissure is also present in adults, it was thought that 

 it might belong to a second pair of ganglia, but this view is not supported 

 by ontogeny, as each of the two ganglia first appear as distinct structures, 

 the apparent division in them arising only secondarily (F. SCHMIDT). [This 

 second commissure appears to be specially developed in the Opisthobranchs, 

 where it is known as the parapedal commissure.] 



The commissures and connectives, so far as their origin has been traced, 

 arise by the growing out of peripheral parts of the ganglia, and the same- 

 origin has been assumed for the peripheral nerves (SALENSKY, HENCHMAN,. 

 v. ERLANGER, F. SCHMIDT, etc.). P. SARASIN, indeed, as above stated, main- 

 tained that the two halves of the cerebral ganglion separated as one connected 

 organ from the ectoderm, and RABL assumed, as we saw (p. 181), that they 

 arose from a common rudiment, the apical plate. It is therefore in any case 

 probable that the cerebral commissure may have arisen from the middle part 

 of the common ectodermal thickening. Such an origin for the commissures 

 and the connectives is on the whole very probable, but is not supported by 

 the observations so far made, indeed, in Bythinia, investigated by SARASIN, 

 the common origin of the two cerebral thickenings has been denied (v. 

 ERLANGER, No. 28). 



The buccal ganglia, as was first shown by SARASIN and as has been 

 confirmed by subsequent investigators, arise as cell -growths of the 

 stomodaeum. The wall of the stomodaeum becomes thickened, and 

 while the cells lying on the inner side retain the cylindrical shape, a 

 number of smaller cells appear on the outer side (Fig. 88 B, bg). 

 These become differentiated into two swellings which lie near the 



* Following LACAZE-DUTHIERS and SPENGEL (No. 122), we distinguish the 

 strands which connect the ganglia of one and the same side as connectives 

 from the transverse strands which connect the right and left halves of a pair 

 of ganglia, these latter being commissures. 



O 



