67 



branchialii' its name. Tlie ctenidium in a fairly deep 

 braiK'liial cavity could not be kept well rinsed among 

 sluggish forms often left uncovered by the tide. The 

 ctenidium, therefore, became inetlicient and has disappeared, 

 the pallial gills being a new and compensating development. 



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

 Perhaps the glandular secretion improves the animal's 

 power of holding on and helps it to withstand desiccation 

 in the exposed spots which these forms t3'pically inhabit. 



No. (10) is a consecpience of having the head always 

 under the shadow of the shell. 



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

 ol)tain a topographical acquaintance with its innnediate 

 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 be said to be directly and closely related to any 

 of the other primitive Prosobranchs, the connection in each 

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

 ancestor. Pleiuvfomaria and the Trochidai have specialised 

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

 muscle, and this is true also of Haliotis, which has adapted 

 itself to creeping about 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 Fissurellidas 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 Fissurellidfe and the shortening of 



