88 Invertebrata. 



opening and closing periodically in the common 

 black slug on the left side of the body, under the 

 edge of the little saddle-like rudimental shell, which 

 in this mollusc is enclosed in the mantle. 



The heads of univalve molluscs bear several 

 organs of sense, tentacles, eyes and ear sacs, the 

 tentacles are long soft feelers, the ' horns ' of the snail, 

 which can be retracted by being involuted, or turned 

 out-side in by muscles. In the common snail the 

 eyes are placed on the extremities of the upper or 

 longest pair of horns, and can be seen as bright black 

 spots. In other molluscs the eyes are either stalked 

 or placed at the bases of the tentacles. The organs 

 of hearing are small sacs placed near the foot, 

 rilled with fluid and containing small concretions. 

 Most univalve molluscs lay their eggs inside little 

 cases often to be met with under stones on the sea- 

 shore. The little ciliated larva has a shell even at its 

 earliest stage, and in some molluscs this shell is lost in 

 development ; in others it is retained and can be 

 seen at the tip of the adult shell as the 'nucleus.' 



Classification. The head-bearing molluscs are 

 very numerous, and are divided into several sub- 

 classes. The first of these includes the little ele- 

 phant's-tooth shells, or Dentalium. This animal has 

 no heart, and is completely enclosed in its mantle, 

 which in the embryo forms at first a minute two- 

 valved shell. Eventually, however, this shell becomes 

 tubular, open at both ends. The second sub-class 

 consists of small molluscs found swimming in the 

 ocean, by means of two large wing-like processes on 

 the upper part of their foot, and hence are named 



