67 



branchiata " its name. The ctenidium in a fairly deep 

 liranehial cavity oould not be kept \\el\ rinsed among 

 sluggisli forms often left uncovered by the tide. The 

 ctenidium. therefore, became inefficient and has disappeared, 

 the pallial gills l)eing a new and compensating development. 



No. (9) is an interesting feature not easy to account for. 

 Perliaps the glandular secretion improves the animal's 

 power of holding on and helps it to withstand desiccation 

 in the exposed spots which these forms typically inhabit. 



No. (10) is a consec^uence of having the head always 

 under the shadow of the shell. 



No. (11) is a specialisation which helps the animal to 

 obtain a topographical acquaintance with its immediate 

 neighbourhood- 



It will thus 1)6 seen that the Docoglossa, though un- 

 doubtedly correctly included among the lower Gastropods, 

 are yet specialised on lines of their own in connection with 

 their adoption of the habit of adhering to exposed surfaces 

 and making limited excursions for the purpose of raking 

 up food. 



They cannot he said to be directly and closely related to any 

 of the other primitive rrosol)ranchs, the connection in each 

 case being due to de-^cent from a not very remote common 

 ancestor. Plcurotomaria and the Trochida3 have specialised 

 on quite other lines as regards the shell, foot, and shell 

 musL'le, and this is true also of Ilaliotis, which has adapted 

 itself to creeping al)Out in chinks and confined spaces. The 

 great contrasts in the respiratory systems and in the general 

 disposition of organs show the distance that separates these 

 forms phylogenetically. The Fissurellid^e have also evolved 

 on quite other lines though they have a shell muscle and 

 visceral hump closely resembling that of the Docoglossa 

 externally. The most striking contrasts are the presence of 

 a pair of ctenidia in the FissurellidcXi and the shortening of 



