158 



HISTOLOGY 



The gill capillaries of Amphitrite are, apparently, one of the excep- 

 tional cases where a blood vessel loses its connective-tissue covering 

 entirely and allows the blood to flow directly among and between the 

 cells of an ectodermal epithelium. It is still possible, however, that a 

 delicate layer of mesodermic cytoplasm, too thin to have been heretofore 

 detected, follows them as their true covering (see Fig. 293). 



The insects have a circulatory apparatus which, considering the high 

 specialization of the group, is remarkably simple. It consists of one 



very large vessel, extending for some distance 

 in the median line of the back. It is mus- 

 cular and serves to carry the blood from one 

 end of the body to the other, and, after a 

 very poor distribution by means of a few 

 short vessels in its anterior region, it receives 

 it again, as it niters its way back, through 

 large sinuses and spaces. It then pumps it 

 forward once more. Separate pumping or- 

 gans in the limbs suffice to carry a stream 

 into the smaller extremities. 



The dorsal heart of a moth larva, Im- 

 perialis, is typical and consists of three layers, 

 or five if we consider the outer and inner 

 layers double (Fig. 141). Each of these two 

 last layers consist of a sheet of flat cells 

 which secrete a homogeneous and elastic 

 cuticle on their exposed surface. The cells 

 of the inner layer are largest in Imperialis, 

 with a large central mass of cytoplasm and 

 the remainder of the cell body so thin that 

 it is sometimes difficult to see it. The outer 

 layer is composed of smaller and somewhat thicker cells with smaller 

 nuclei. This layer, on the two outer, lower quadrants of a section of 

 the vessel, is evaginated into a series of processes which come into ex- 

 tremely intimate relations with a plexus of tracheal capillaries. In 

 fact, the finest air capillaries enter in great numbers into the substance 

 of the process. The outer cuticle is not in full thickness over these 

 processes. The nuclei of the layer are found in the processes also. 



The middle layer of the vessel consists of a single or double layer of 

 muscle fibers. They are clearly striated and have no distinguishing 

 cardiac features except, perhaps, a slightly shorter sarcous element. It 

 is hard to distinguish the nuclei of these fibers from the nuclei of the 

 outer and inner layers which bear the cuticle. 



Amphioxus has a well-defined blood-channel system that is lined, in 



mus.f. 



FIG. 141. Part of a transverse 

 section of the heart wall of 

 an Imperialis larva, cu. cuti- 

 cle, outer and inner ; />., pro- 

 cess on outer side. mus. /., 

 muscle fiber, x 900. 



