NO. 6(I92I) COMMON MOLLUSCS OF SOUTH INDIA I5I 



is noteworthy that the original form of branchiae has not re- 

 appeared, a wholly different part'of the body being specialized 

 anew for the purpose, a fact, which, Pelseneer remarks, illustrates 

 the irreversibility of evolution. 



Both Limnaea and Planorbis have the habit of crawling upside 

 down beneath the surface of the water, the foot gliding along 

 exactly as if it were moving along a sheet cf glass; the well 

 known phenomenon of the surface tension of fluids is the expla- 

 nation of this miracle. Their spawn is deposited in colourless 

 gelatinous masses on stones and water weeds. In hot weather 

 when tanks and streams dry up, they aestivate in the mud in the 

 same way as Pachylabra and Vivipara. 



CLASS Ill.-SCAPHOPOPODA. 



Standing apart from all other Gastropods is the little group of 

 the Scaphopods, in many ways so specialized as to appear degene- 

 rate for there are neither eyes, nor tentacles nor any distinct head 

 present; neither is there any gill nor any true "heart." The only 

 well-known species are the TuSK-SHELLS {Dcntaliidac). In these 

 the edges of the mantle lobes unite to form a complete tube, and 

 as a consequence tlie shell is tubular. At one end, the posterior, 

 it is narrow, widening gradually to the other end, through which 

 protrudes the short beak-shaped foot and a number of long fila- 

 ments. The tusk-shells belonging to the genus Dentaliiiin are often 

 very common in sandy ground in depths of a few fathoms 

 around our coasts; their empty shells are often cast up ashore. 

 The most abundant species in Indian waters {Doitaliiiiu octogoniDii) 

 has a very pretty curved shell, snowy white, with eight grooves 

 running down the entire length, giving an octagonal outline in 

 transverse section. It grows to a length of an inch and a half. 

 Another species, very rare, grows to a length exceeding three 

 inches and is stout in proportion. This species which [ have seen 

 on only a few occasions, has a shell mottled with shades of dark 

 red, a very handsome shell. All live buried obliquely in the sand, 

 pointed end upwards. In this there is a small perforation and it 

 is through this that the water required for breathing passes in. 

 The food of the tusk-shells consists of foramJnifera and minute 

 bivalves. 



