552 



THE INVERTEBRATA 



minutes in Acmaea so that it can hardly be brought about by differ- 

 ential growth. Muscular contractions must play their part. 



The large majority of gasteropods belong to the order which ex- 

 hibits torsion in full development. It is called Prosobranchiata, 

 because of the anterior position of the gills, or Streptoneura, because 

 of the coiled visceral loop. The periwinkles, whelks and limpets of 

 our shores, the freshwater Paludina, and many others belong to it. 

 The order may, however, be divided into two groups, a primitive one 

 in which the two ctenidia and consequently the two auricles are pre- 

 served {Diotocardia represented by Patella, Fissurella and Haliotis) 

 (Fig. 380 A-C), and a more specialized one in which the right (primi- 

 tive left) gill, its auricle and even the right kidney have disappeared 



M. 



max. 



Fig. 379. Diagram to illustrate torsion, when seen from above. A, Ancestral 

 gasteropod. B, 90° of torsion. C, Torsion completed (180°). After Naef. 

 an. anus; au. auricle; ce.g. cerebral ganglion; M. mouth; max. mantle cavity; 

 pa.g. parietal, ped.g. pedal, pl.g. pleural, vis.g. visceral ganglia ; v. ventricle. 



{Monotocardia, represented by Littorina, the periwinkle, and Buc- 

 cinumy the whelk) (Fig. 380 D). Some of the Diotocardia, like Trochus, 

 are in an intermediate state in which, though the right gill has dis- 

 appeared, there is still a rudiment of the corresponding auricle. Be- 

 sides this fundamental difference, there are others. For example, in 

 the Monotocardia, special generative ducts are developed (cp. also the 

 penis of the male Buccinum), while in the Diotocardia, the generative 

 organs open to the exterior through the right kidney. 



It is possible that the disappearance of the organs of one side is to 

 be regarded as the consequence of the processes concerned in torsion 

 and that in the Diotocardia the phenomenon cannot be regarded as 

 having reached its climax. On the other hand, there is a large division 



