Structure of Liinulus Heart Muscle. 405 



on the dorsal side. The posterior end is attached to the carapace 

 by a broad sheet of connective tissue. The outer surface of the heart 

 looks as if it were longitudinally striated, an appearance which is 

 due to a layer of elastic tissue strands. 



Microscopic Histology. 



In cross section the heart is somewhat triangular in outline and 

 may be seen to consist of three layers. (See Fig. 1.) The median 

 layer (Fig. 1, b) is a fine, dense, connective tissue support, termed 

 the basement membrane (Patten and Redebaugh, 1899). Outside 

 of this is the longitudinal layer of elastic tissue fibers mentioned 

 above. Inside the basement membrane is the muscular layer which 

 consists of branching, anastomosing strands, arising from the base- 

 ment membrane and running mostly in a circular direction. The 

 muscular layer is thickest at the angles of the heart and thins out 

 over the aortic valve. It is with this layer that v/e are most con- 

 cerned. 



By reference to Fig. 1 the general characteristics of the muscular 

 heart wall can be understood. Trabeculse of striated muscular tissue 

 arise from the basement membrane, branching and anastomosing 

 until a spongy heart wall is formed. 'Ro membrane limiting the 

 lumen corresponding to the endocardium of the mammalian heart 

 is present. In only one region is the lumen lined and that is at 

 the aortic valve where for a short space the muscle layer lies be- 

 tween two connective tissue layers, the basement membrane above and 

 the tissue of the valve beneath. 



The blood circulates freely around the strands of the heart, pass- 

 ing into all the interstices and crevices, thus bathing the entire mus- 

 culature. This arrangement for securing nutriment seems to be 

 quite sufficient. A small amount of connective tissue appears between 

 the trabecular, especially in the thicker parts of the heart wall. This 

 can easily be traced back in origin to the basement membrane. It 

 would seem that capillaries might enter along with this tissue, but 

 as far as can be determined they do not do so, unless it bo in the 

 deeper parts immediately adjacent to the basement membrane. 



A small fragment of the heart wall macerated in 20 per cent nitric 



