PRIMARY FUNCTIONS OF THE ORGANISM 39 



traction which it is of advantage to the organism to 

 have produced. In the vertebrates the tissues that 

 carry out slow, rhythmic contractions, such as those 

 of the intestinal walls, are made up of small, narrow, 

 spindle-shaped cells (fig. 15) with a single nucleus 

 and a delicate longitudinal striation. The skeletal 

 muscles and those throughout the animal series in 

 which rapid or " voluntary " movement is produced 

 are very highly differentiated (fig. 15 c). In the 

 specialization of their substance along the line of 

 contractility they have lost the function of food- 

 taking, and to a great degree, though not entirely, 

 that of conduction and general irritability. 



Specialization in Conducting Organs. A par- 

 ticular kind of irritability, however, has come into 

 play in connection with another sort of cells called 

 nerves. Nerve-cells represent another line of spe- 

 cialization. Here the function of specific irritability 

 and conduction has been developed, until, in com- 

 pensation, the functions of contractility and nutri- 

 tion have entirely disappeared. 



An experiment of Professor J. Loeb's demon- 

 strates in an interesting way the performance of the 

 same function by a highly specialized tissue and by 

 a less specialized one. One of the group of degen- 

 erate animals called Tunicates is provided with 

 two siphons (fig. 16) or passages, through one of 

 which the water passes into the saclike body and 

 through the other of which it flows out. Midway 

 between the two is situated the nervous system, 



