8 B. T. LOWNE ON THE MICROSCOPE IN THE 



these fibrillar appears to be made up of a number of alternate cylin- 

 ders of greater and less opacity. The non-striated muscles consist 

 of more or less elongated fusiform cells, exhibiting distinct nuclei, 

 or of long fibres bulging at intervals, and exhibiting nuclei in the 

 swollen spaces. 



In the Vertebrata, these two forms of muscle are constant in their 

 occurrence ; the former in all the muscles of the body, which serve 

 the purpose of locomotion as well as all ordinary voluntary, or re- 

 flex acts ; the latter constitute the muscular coats of the viscera 

 and blood vessels, except the heart, which has its muscular walls 

 composed of the striated variety of muscle. 



In the Mollusca and Insecta the case is however far different ; in 

 the former the non-striated, and in the latter the striated varieties 

 enter into all the muscles, almost without exception ; thus, the walls 

 of the stomach and intestines of insects exhibit the striated kind, 

 whilst the elaborate muscular system of the Cephalopoda is en- 

 tirely composed of non-striated muscle. 



In the Annelida again, which forms a large division of the 

 Annulosa, and which present many characters which show their 

 close affinity to insects, the non-striated muscle only exists, — 

 whilst the Ascidians, a group of the Molluscoida, according to the 

 best authorities, possess striated muscular fibres. 



This irregular distribution of striated and non-striated muscle is 

 worthy of careful consideration, but especially in relation to the 

 origin of species ; much has yet to be done in working out the re- 

 lation of the one kind of fibre to the other. To say the least, it is 

 a fact yet to be accounted for by those who believe in the descent of 

 several types from a common ancestor. 



Other facts of a kindred nature might be carefully worked out, 

 and the results of such investigation may have the most important 

 issue. 



The close resemblance of the Spermatozoa of the most widely 

 separated groups of animals, and their great diversity of form and 

 size in nearly allied types, are facts, the import of which are at 

 present not understood. The apparent identity of nerve cells and 

 fibres in the most diverse types, together with the strange manner 

 in which the different kinds of muscular fibres are distributed, the 

 absolute unity of structure existing side by side with the greatest 

 diversity of type and form, are all facts bearing upon the same 

 great problem. I commend them to your notice, gentlemen, feeling 



