TYPE MOLLUSC A. 



297 



this simple form, its margin becoming fringed or tentaculate, 

 or else it may be reduced to one or more separate lobes or 

 tentacular processes on either side of the bod}". Opening 

 upon the surface of the foot is frequently to be found a so- 

 called "foot-gland " which secretes a sticky mucous fluid and 

 is comparable to the byssus-glaud of the Pelecypoda (q. v.). 



FIG. 132. Buccinum undatum. 

 op = operculum. si sipho. 



The respiratory organs (Fig. 133, ct) consist in typical cases 

 of a single pair of pinnate branchial plumes lying in the 

 mantle-cavity, but, as has already been mentioned in connec- 

 tion with the rotation of that cavity, one of these structures is 

 very frequently aborted. Other changes, however, also occur, 

 such, for example, as the fusion of the central axis of the 

 branchial plume throughout its entire length to the inner 

 surface of the mantle (Haliotis), or the disappearance of the 

 pinnae from one side of the plume in connection with such a 

 fusion (Sycotypus, Fig. 133). In some forms accessory bran- 

 chiae may be produced as folds of the mantle, richly supplied 

 with blood, and their development may be carried to such an 

 extent that they may entirely supplant the branchiae proper 

 (Patella). From such a condition as this a passage is not dif- 

 ficult to such a condition as is found in the air-breathing Gas- 

 teropods (Pulmonata) in which the entire inner surface of the 

 mantle serves a respiratory purpose, an interchange of gases 

 taking place between the air contained in the mantle-cavity 

 and the blood which is richly supplied to the mantle. 



The musculature of the body-wall does not as a rule pre- 

 sent the Aunelidan arrangement in layers, as in some Amphi- 

 neura, but usually are irregularly arranged as dorso-ventral 

 and oblique bands traversing the schizoccel. Special muscles, 



