ANATOMY AND LIFE HISTORY OF MOLLUSCA. 117 



by means of a gill-plume ; but if the Radula of the first-named 

 species is examined in its natural position, it will be found to be 

 exactly like that of the common periwinkle of the British coast. 

 It is a long, slender thread, five or six times the length of the 

 whole animal, and strange to say it lies sheathed in membrane, 

 and neatly coiled up like a piece of rope at the back of the head. 

 This peculiarity is shared by all the periwinkles known to me on 

 the Australian coast, though extending to different genera or 

 sub-genera such as Hisella, Tectarius, and Littorina. Without 

 stopping to enquire into this peculiarity, for which many reasons 

 might be given, I may say, that the structure of the Radula itself 



is that of the British periwinkle expressed in the formula . 



There is a figure in Woodward's " Manual of Mollusca " (2nd 

 edit., p. 252), of the European periwinkle, which does not quite 

 correspond with any of our Australian species. 



In Trochocochlea tceniata, Lam., the type of Radula is entirely that 

 of the turbinate Gastropods or nacreous trochoid shells, the 

 Rhipidoglossa of Troschel: that is to say the central large tooth 

 with five laterals and an indefinite number of lanceolate uncini 

 decreasing from the centre to the edge of the Radula, until they 

 become hair-like hooks set together like the plumes of a feather. 



In Acmcea septiformis, Q. and G., or A. marmorata, Tenison- 

 Woods, there is a gill-plume at the back of the neck as already 

 stated; but the type of the shell is the conical one of Patella and 

 that also is the type of the Radula. This is a long, deep brown 

 ribbon, with pairs of long central teeth and no uncini. (See pi. iv., 

 fig. 3. Patella tramoserica, pi. iii., fig. 2.) 



So that in these three instances we have illustrations of two 

 different organizations, in which the organs seem to be associated 

 according to no definite rule. In the one case the Radula would 

 seem to follow the structure of the branchial apparatus ; in the 

 other a certain form of Radula seems to belong to a certain form 

 of shell, while the branchial arrangements are quite different. 



Branchial Organs. — The type of the gills or branchiae, which 

 in some form or another are placed in the cavity between the 

 mantle and the foot, is a series of filaments forming separate 

 lamellae. I just refer to the fact that the gill is a differentiation 

 of the integuments, and is superficial primitively, becoming placed 

 in a special cavity by being covered by another fold of the 

 integument. Each gill-lamella is developed from a row of 

 processes growing close to one another and remaining separate 

 occasionally, but in most cases growing together and forming a 

 plate. The union, however, is not complete. Eine clefts exist at 

 intervals between the filaments through which the water passes. 

 ■" These filaments are not simple prolongations, but loops, so that 



