CIRCULATOEY SYSTEM IN ARACHNID A 159 



beginning with the posterior edge of the sixth tergite. The 

 ligaments of the heart are cut close to the hj^odermis with 

 a sharp scalpel, as otherwise the heart would sustain injury. 

 Freshly precipitated carmine as injection fluid proved to be 

 quite satisfactory. Not only the large vessels become injected 

 to their end near the base of the. claws in the legs, but many 

 ramifications of pedal arteries appear dark red. The injected 

 specimen is next fixed in 95 per cent alcohol, dehydrated in ab- 

 solute alcohol, and cleared in cedar oil, in which it becomes 

 sufficiently transparent for further preparation. All organs ob- 

 structing the view are now carefully removed with the aid of 

 two needles under a binocular dissecting microscope and the 

 entire circulatory system exposed to view. 



Usually the circulatory system in scorpions is described as 

 consisting of a dorsally situated heart which gives rise anteriorly 

 to the cephalic aorta and posteriorly to the posterior aorta. The 

 heart itself is said to consist of eight chambers with a pair of 

 ostia each, or eight pairs of ostia altogether, typically one pair 

 for each segment of the body. The cephalic aorta is described 

 as giving rise to a pair of arteries near its base, and a little further 

 to another pair. The latter assume a do\\Tiward course, pass 

 on each side of the oesophagus, forming a ring from which six 

 pairs of arteries are said to be given off to the appendages, while 

 a single median supraneural artery runs from the ring backward 

 above the nervous system. The usual description of the finer 

 ramifications, as well as of the arteries given off by the heart, 

 is irrelevant to our purposes and may be entirely omitted. 



The microscopical structure of the heart seems in all Arthro- 

 poda to be more or less the same. Its wall is composed of three 

 layers. The outer layer, the adventitia, consists of connective 

 tissue. The heavy media or muscularis is formed either by a 

 spiral muscle or by symmetrically arranged semicircular mus- 

 cle fibers which meet in the middorsal and midventral line, as 

 has been described by Bergh for insects. The inner layer or 

 intima is, whenever present, nothing but a very thin transparent 

 membrane which, according to Verson, may be nothing but the 

 sarcolemma of the muscle fibers of the media. 



