478 THE INVERTEBRATA 



completing a figure of eight. The whole process takes only two or 

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

 differential growth. Muscular contractions must play their part. 



The large majority of gasteropods belong to the order which 

 exhibits 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. 356 A-C), and a more specialized one in which the right (primi- 

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



Fig. 354. To show torsion in Paludina vivipara. After Naef. Embryos seen 

 from the side (A, B, D) and behind (C). A, Almost symmetrical stage, with 

 mantle cavity behind but anus twisted a little to the right. B, Stage showing 

 90° of torsion, mantle cavity and anus to the right. C, Torsion at almost the 

 same stage as B. D, Stage showing 180° of torsion and the adult condition. 

 an. anus; m.c. mantle cavity; op. operculum; vm. velum. 



(Mowo^ocflr^/fl, represented by Littorina, the periwinkle, and Buccinum, 

 the whelk) (Fig. 356 D). Some of the Diotocardia, like TrochuSy are in 

 an intermediate state in which, though the right gill has disappeared, 

 there is still a rudiment of the corresponding auricle. Besides this 

 fundamental difference, there are others. For example, in the Mono- 

 tocardia, 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 



