Dr. G. Meissner on the Genus Mermis. 423 



animals ; vvc will now turn and glance at some of the important 

 histological points, as wrought out by Meissner. 



Cutaneous System. — Omitting the very full details given of 

 the structure of the skin in these animals, its composition of three 

 distinct layers, &c, we will allude only to the fact that chitine 

 enters into its formation. This fact is important as corrobora- 

 tive of other observations. Chitine was formerly supposed to 

 belong exclusively to the teguments of the Arthropoda, being 

 particularly prominent in the skin of insects ; but recent chemi- 

 cal analyses of the teguments of lower animals show that it 

 occurs in nearly every class of the Invertebrata*. It can there- 

 fore no longer be regarded as having diagnostic characteristics 

 for certain classes, but sustains relations to the external dermic 

 skeleton of the Invertebrata generally, analogous to those of 

 bone in the four classes of Vertebrata. 



Muscular System. — This was found quite developed, and it is 

 a singular fact that all the muscles have a longitudinal direction. 

 Transverse muscles do not exist. But Meissner has indicated a 

 histological feature of the muscular tissue in these animals, which 

 deserves notice. It is well known that striated muscular fibre is 

 rather limited in its distribution among the Invertebrata. We 

 have not observed it below the Articulata, and have regarded it 

 as actually absent in the remaining classes — the Cephalopoda, 

 Cephalophora, Acephala, Annclides, Turbellaria, Helminthes, 

 Echinodermata, Aealephse, and Polypi. Now, we have hitherto 

 supposed from observations that the fibre being the true embryo- 

 logical element of muscle, a further division into fibrillse occurred 

 only in the higher form of this fibre, the so-called striated muscle ; 

 in other words, that a fibrillated structure of muscular fibre was 

 found only in the striated form. But Meissner describes the 

 fibre of Mermis as readily capable of being split up into longi- 

 tudinal fibriUse of the most regular and delicate character, and 

 yet neither these fibres nor fibrillse are properly transversely 

 striated. He remarks however, that an appearance like stria- 

 tum is sometimes observed during a wave-like contraction of 

 the nbret. Results of this character which the more careful 

 research of the present day is developing, in the study of the 

 lower animals especially, fullyindicate that the subject of muscular 

 tissue is not well understood as to its manifold variations of 



* Besides the present case we would refer to the following : Gruhe, 

 Miiller's Arch. 1 848, p. 461, and Wiegmann's Arch. 1850, p. 253 ; Schultze, 

 Beitr. zur Natnrgesch. d. Turhellarien, p. 33 ; and Leuckait in Siebold 

 und Kolliker's Zeitsch. 1851, p. 192, and Wiegmann's Arch. 1852, p. 22. 



t We suspect it is this same wave-like aspect that has been often mistaken 

 for striation in the muscles of some of the lowest animals, thereby leading 

 to no little discrepancy among observers in their statements. 



