OBSERVATIONS ON STRIPED AND UNSTRIPED MUSCLE. 89 



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 developed 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 movements 

 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 Earth- 

 worm the muscle is unstriped. This can, I think, be explained as 

 follows. These so-called " hearts " represent the earliest and most 

 primitive form of heart in the animal kingdom, being simply local 

 hypertrophies of the blood vessels which perform rhythmic contrac- 

 tion. 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 

 Patelbi, Snail, &c, which have to perform much more rapid and 

 regular contractions than the "hearts" of the worm, that we find 

 striped muscle developed. 



I may here state that I have not yet been able to determine the 

 nature of the connection between the network of striped muscle and 

 the nerve end-plate, which must exist if the combined results of 

 Betzius and Bremer are correct. This I hope to do in a subsequent 



