ON THE MICROSCOPE IN THE CLASSIFICATION OF ANIMALS. 7 



organs of the invertebrata, especially where they differ materially (as 

 they usually do), in their structure and appearance from those of ver- 

 tebrates. In fact, I know of no other sure guide to function except 

 histological structure. 



It is true that the histological tissues of the lower forms of 

 life frequently exhibit characters which belong to the same 

 structures in higher animals, whilst yet in an undeveloped or 

 embryonic condition. For instance, muscles appear to be developed 

 by the fibrillation of proto-plasm, and in such forms as the higher 

 Hyclrozoa, the muscles consist merely of proto-plasm, exhibiting 

 indications of fibrillation. The existence of nuclei in the muscles 

 of insects has been ascribed to a similar undeveloped condition, but 

 must, I think, be attributed to rapidity of growth rather than to 

 any arrest of development, since the muscles of insects exhibit the 

 utmost perfection of structure, and the nuclei are often absent when 

 the insect has reached maturity. 



Rapidity of growth seems to have an important influence on the 

 character of all histological tissues, and as far as my observations 

 go, tends to increase the size of the ultimate elements of the fabric 

 very materially ; for instance, the muscular fibres and gland cells 

 of the larva? of insects, are often nearly ten times as large as the 

 same structures in the perfect state. It has been repeatedly stated 

 that the muscles of insects have larger fibres than those of verte- 

 brates, — such is the case in the rapidly growing tissues of the 

 larva, but I have repeatedly found muscular fibres in insects as 

 small as ^-qVo of an inch in diameter, which is nearly as small 

 as any known fibres. 



I do not know any more important tissue than muscle, nor any 

 which presents so complex a problem in its distribution and develop- 

 ment in different animals. 



Two kinds of muscle have usually been described ; these are 

 known to anatomists as the striated and non-striated varieties. 

 The structure of striated and non-striated muscle differs very 

 materially, the former consists of bundles of fibrillar of very minute 

 diameter, bound together in a membranous sheath so as to form a 

 muscular fibre, a number of muscular fibres constituting a muscle. 

 The fibres exhibit regular transverse stria? ; and when the fibres are 

 broken up by the rupture of the myolemma or sheath, the fibrillar 

 of which they are composed present the appearance of minute 

 fibres, presenting alternate dark and light spaces, so that each of 



