66 HISTOLOGY 



perform it by the development of strong fibrils in their cyto- 

 plasm. 



Figure 67 shows a bit of shell resting on the columnar epithelium 

 that formed it. In the right-hand portion of the figure is seen this epithe- 

 lium in an undifferentiated state, with its base resting on a delicate layer 

 of connective tissue. The exact boundary between the base of the epithe- 

 lium and this connective tissue is difficult to detect. In the left-hand part 

 of the figure can be seen the same structures, but somewhat modified, 

 owing to the fact that some muscle fibers are attached to them at this 

 point. 



Both the connective-tissue cells and the epithelial cells have modi- 

 fied their structure and united to act as a short ligament. The fibrils 

 which were weakly developed in the undifferentiated epithelial cell 

 have become heavy and strong, and lie in parallel positions in this differ- 

 entiated one. The weak fibrils that run in irregular courses and partly 

 parallel to the shell in the first set of connective-tissue cells have become 

 stronger and assumed a position of greatest efficiency, at right angles 

 to the shell and in line with both muscle and epithelial fibrils, in the tissue 

 seen in the second group. As in most connective-tissue cells, the actual 

 boundaries are too complicated and weakly marked to be seen. 



These three sets of fibrils, the epithelial cell fibrils, the connective- 

 tissue cell fibrils and the muscle fibrils, have been joined individually 

 into a set of long and apparently continuous fibrils, of which the first 

 two portions form the passive ligamentous part, while the last acts as 

 the actively contractile portion. The strain thus comes, in all three 

 of these cells, not on the cytoplasm of the cells but on special cell-organs 

 which have been formed by the cytoplasm to meet it. 



This mode of muscle attachment is found throughout the lobster's 

 body. In some places the connective-tissue elements are so small as to 

 be apparently absent, and it would seem possible that in some attach- 

 ments they were absent altogether and the muscle was joined directly 

 with the epithelium. This condition is clearly true in some lower Crus- 

 tacea, and has been figured by Schneider in Branchipus. 



Study in this connection : the mode of attachment of the closing muscle 



to the shell in a Unio; the attachment of an Anomia to a rock and the 



attachment of muscles to the head-cartilages in the squid or Octopus. 



Technic. Use the same methods that were used for the simpler 



tensile forms. 



LITERATURE 

 Read the same articles that were suggested for the preceding parts. 



