MOLLUSCA 



161 



mtside, the other into the pericardium, a reduced body 



ivity in which the heart is situated. 

 Reproduction is sexual, that is it takes place through 



le union of two dissimilar cells, the egg cell and the 

 sperm cell. Both eggs and sperm may be found in the 

 ie individual (land snail), but the sexes are usually 



>parate. After fertilization a larval form is developed 



r hich through a series of changes called a metamor- 

 )hosis becomes like the parent form. 



The group of mollusca is divided into five classes : 

 jight-valved forms in which the nervous system is much 

 Amplified; very primitive one- 

 ralved forms in which the shell 

 tubular ; bilaterally sym- 



letrical, two-valved forms with 



>aired organs, without head or 

 appendages ; one-valved forms 

 Avith coiled shell which have a 

 head, bearing eyes and tentacles, 

 a creeping foot, and unpaired 

 organs, no gills, the walls of the 

 mantle cavity acting as lungs; 

 and more highly developed 

 forms having a single shell or 

 none, unpaired mantle and 

 mantle cavity, one or two pairs 

 of gills, one or two auricles, one or two pairs of branchial 

 hearts, an ink sac, and a well-developed head bearing 

 eyes and tentacles (Fig. 91). By rhythmically forcing 

 water out of the siphons they are able to swim. To this 

 group belong the squid and the devil fish. 



Fig. 91.— The squid (ventral view). 

 (From Hertwig, after Hoyle.) 



