THE RAT-FLEA, CBRATOPHYLLUS FASCIATUS BOSC -tG 1 



and very delicate layer of muscular tissue, in the form of a 

 network (PI. 32, A, B and C). The individual muscle-cells 

 are branched like ganglion -cells, and their processes anastomose 

 to form the network. Some nodes of the network are formed 

 merely by the union of two or three such processes, while other 

 nodes are formed by the body of the cell, and contain the 

 cytoplasmic cell-body with a nucleus. The processes themselves 

 are transversely striated, and form the actual muscle-fibres. It 

 can be seen that at a cellular node of the network the striated 

 fibres pass right through the body of the cell and come out 

 on the other side, their striation and individuality becoming 

 slightly less distinct in their passage through the cytoplasm of 

 the cell. 



Remembering that I had seen muscle-cells of a somewhat 

 similar type in the " crop " or " sucking stomach " of the tsetse- 

 fly, an organ which is morphologically a diverticulum of the 

 oesophagus, I made a preparation of the crop of a common 

 house-fly and found a musculature of a very similar type 

 (PI. 32, D, E). The main differences are, first, that the cells 

 are on a much larger scale of size, requiring lower powers of the 

 microscope for their study ; secondly, that the muscular network 

 has a definitely rectangular arrangement, those fibres which run 

 in certain directions being considerably thickened, and connected 

 with one another by delicate fibres running across at right 

 angles. In some parts the thickening of these longitudinal fibres 

 is much more marked than in others. All the fibres, even 

 the thinnest, show the characteristic transverse striation very 

 distinctly. 



The resemblance of the muscular network in the two cases 

 raises some interesting points of phylogeny. In the first place, 

 it should be noted that a contractile network is the most 

 efficient arrangement for the contraction of a bladder, since it 

 gives an even contraction in all directions. The muscles of the 

 human urinary bladder are also arranged in a network, but on a 

 much larger scale than those described here, since the strands of 

 the network are not outgrowths of individual cells, but are made 

 up of thick bundles of contractile cells. It is therefore not 

 surprising to find a network in the contractile elements of an 

 organ such as the crop of the house-fly. On the other hand, it 

 is rather remarkable to find it in the oesophagus of the flea. 



