Logan.] The Invertebrates, Introduction. 439 



bers are provided with a calcareous tube or shell into which the 

 soft parts are drawn. Sometimes these tubes are free, but more 

 often they are attached to some submarine object. The tubes 

 are usually calcareous but sometimes they are composed of 

 sand-grains or pieces of shell cemented together. 



Mollusca. 



This subkingdom is represented by a great diversity of ani- 

 mal organisms, making a specific definition next to impos- 

 sible. The members of the mollusca differ physiologically, 

 in that some have none or poorly developed heads and hearts, 

 while others have heads and chambered hearts. Some are 

 naked ; some have a gristly covering, while others have either 

 a double or single valve or shell composed of calcareous mat- 

 ter. The individuals range in weight from a few grains to 

 400 or 500 pounds. A marked difference is found in their man- 

 ner of growth. Some, like the Radiolites, grow in colonies ; 

 others, like the majority of the Ostrese, grow independently. 

 They possess a soft, non-segmented body, and have the power of 

 secreting matter from the surrounding water for the construc- 

 tion of their own shells. They also possess a thick membrane, 

 called the mantle, which secretes the calcareous matter for the 

 shell, and also determines the shape of the shell. Some of the 

 members of this subkingdom are endo-skeletal, i. e., the horny 

 or calcareous part being on the inside ; others are exo-skeletal, 

 with the horny or calcareous part on the outside. Many mem- 

 bers of this subkingdom are provided with numerous mouths 

 or suckers for capturing their food. It is said " the Clio borealis 

 has no less than 360,000 suckers' attached to the wing-like 

 organs which spring from its head." The pearly nautilus, 

 the cuttle fish, the octopus, the sea squirts, the oyster and the 

 clam are all representatives of this subkingdom. The Creta- 

 ceous seas of Kansas swarmed with mollusks of all kinds, from 

 the small Ostrea coagesta to the large Radiolites maximus. 



