OBSERVATIONS ON STRIPED AND UNSTRIPED MUSCLE. 93 



movements are more rapid, and in these we find the network 

 developed. Also in the hearts of these animals which perform 

 rapid and regular contractions I find the network present, at 

 any rate in the case of Patella. 



In the Pecten we have a Mollusc which differs from the 

 majority of its class by performing rapid movements by the 

 contraction of its adductor muscle, and here we find the 

 network present. This is a most important fact in favour of 

 the view that the peculiar network of striped muscle is deve- 

 loped when rapid movements are to be performed; for here 

 we have the Mussel and the Pecten, both belonging to the 

 same division of the Mollusca, and both having adductor 

 muscles moving the valves of the shell. In the Mussel the 

 adductor muscles only act at irregular intervals and compara- 

 tively slowly; but in the Pecten they perform rapid and 

 frequent contractions when the animal swims. In the Mussel 

 we find unstriped muscle, but in the Pecten the network of 

 striped muscle is present. 



In the majority of Arthropods and Vertebrates the move- 

 ments are chiefly rapid and of frequent occurrence, and in 

 these groups there is a wide distribution of striped muscle. 



It is quite possible that in some animals of sluggish habits, 

 such as some adult insects, the presence of striped muscle may 

 be due to inheritance. 



We should expect on this view to find striped muscle present 

 in all well-developed hearts, since they execute rapid and 

 regular contractions. However, in the so-called " hearts " of 

 the Earthworm the muscle is unstriped. This can, I think, be 

 explained as follows. These so-called " hearts n represent the 

 earliest and most primitive form of heart in the animal 

 kingdom, being simply local hypertrophies of the blood-vessels 

 which perform rhythmic contraction. Now, the muscle of the 

 blood-vessels is unstriped, therefore we should scarcely expect 

 to find striped muscle in what are simply local hypertrophies 

 of those vessels. Moreover, the contraction of these " hearts " 

 is slow and peristaltic in nature. It is only when we come to 

 the more highly developed hearts, such as those of the Patella, 



