330 ORGANIC GROWTH sec. 



muscle only occurs, but also always occurs, where very active 

 muscular contractions take place. Thus the visceral mus- 

 culature of Vertebrata is everywhere composed of smooth 

 muscles, with the exception of the heart, which is composed 

 of striated muscle-cells — the former, therefore, occur at every 

 part of the viscera where movement is slow, the latter alone 

 where it is extremely active. A no less significant fact is 

 the foUowincj : leaving aside the smooth muscle-cells which 

 occur in the mesoderm of sponges, the neuromuscular cells 

 form the commencement of the development of the muscula- 

 ture in the zoophytes. But in spite of their primitive 

 character the muscular elements of these cells, in cases in 

 which they produce vigorous and rapid motion, are trans- 

 versely striated, while in other cases they are smooth. They 

 are smooth in Hydra, but striated on the lower surface of the 

 bell in the Medusae, which, in order to effect locomotion, 

 respiration, and to obtain food, is in constant motion. 



The inactive Mollusca have generally smooth muscle- 

 cells — striated muscle occurs only in one position, namely, 

 in the adductors of those Lamellibranchs which can swim 

 swiftly in the water by the rapid flapping of their valves, as 

 butterflies fly by the flapping of their wings, e.g. Pecten. But 

 in relation to this question the adductors of Anodon are still 

 more worthy of attention : in these I find the first traces 

 of striation, but it is not yet a permanent morphological 

 character ; it appears only temporarily and only at separate 

 parts of the fibre, merely as the outward sign of activity. 



It is usually stated that the Arthropods have striated 

 muscle-fibres only. It is true that in these actively-moving 

 animals the striation is especially well marked, and it is in 

 them that the subdivision of the fibril into " prisms " has 

 been described, this supposed constitution having been 

 afterwards attributed to all muscle -fibres as though the 

 " prisms " were the elementary parts of the fibrils everywhere. 



