T8o DESCRIPTIONS OF PREPARATIONS. 



arteries serve to distinguish it from the similarly unilocular and non- 

 vasiform heart of some Entomostraca, while both these forms are in turn 

 distinguished by their compressed shape from the vasiform structure found 

 in Squilla and most Arthrostraca as well as in other classes of Arthropoda. 



The tissue of the heart consists, according to Dezso, of muscle cells. The 

 striated substance is present only on one side of the cells as in Nematoda, and the 

 cells are so grouped that their striated sides form the axis of a cord of cells. The 

 nuclei are numerous and nucleolated. This histological condition must be regarded 

 as a persistent state of what is an embryological phase in the development of 

 muscular tissue in Arthropoda. The cords formed by the cells cross in all directions, 

 and the heart-walls are spongy in texture. The inlets ' represent blood-spaces in 

 the walls leading into a ventricular cavity.' The pericardium is non-muscular. It 

 lies upon the heart, and consists of elastic connective tissue with a few scattered 

 nuclei, and an outer layer of ordinary connective tissue. Bipolar ganglion cells, each 

 in its own capsule, are to be found in the posterior half of the dorsal surface of 

 the heart, frequently in groups of three or more. The muscular tissue of the heart 

 contains myohaematin, as does that of the Lobster and the Crab, according to 

 MacMunn. 



A medium-sized artery possesses three coats a structureless intima, a middle 

 coat composed of circular fibres, probably of connective tissue as they are not 

 striated, and a homogeneous adventitia with numerous nuclei, regularly arranged. 

 The intima disappears (?) in the smaller vessels. The middle coat is best marked 

 in the large and medium-sized vessels, while the adventitia increases in thickness, 

 and becomes both lamellate and fibrillate in the medium-sized vessels. Many of 

 the arteries possess a sheath of cellular connective tissue, which is best seen in the 

 superior abdominal artery. 



The capillary system is well developed, and forms networks : one of the easiest 

 to demonstrate is the one on the surface of the supra-oesophageal ganglion. The 

 capillary has a structureless wall with an oval nucleus here and there. Its cavity 

 is very narrow hardly admitting a blood-corpuscle. 



The venous channels, according to Haeckel, have distinct walls composed of a 

 thin plate of homogeneous nucleated connective tissue intimately united with the 

 connective tissue coats of the various organs. They are always, according to him, 

 well-defined channels, not irregular spaces. However this may be, they must, 

 strictly speaking, be considered as constituting the peri-visceral cavity, rather than a 

 system of vessels independent of that cavity. The venous space in the sternal 

 canal is connected to the spaces which lead to the branchiae. The efferent branchial 

 canals are distinct vessels, six in number, which ascend the walls of the thorax and 

 open with widened apertures into the pericardial sinus. 



The blood-spaces of the branchiae are inter-cellular spaces, see p. 183. It is 

 possible that, as in Phyllosoma, &c., the blood circulating in the branchiostegites may 

 return to the heart without passing through the branchiae. 



The blood-corpuscles are colourless and amoeboid. The plasma contains 

 haemocyanin (p. 1 1 2), and a red lutein, or lipochrome known as tetronerythrin, both 

 of which are found also in the blood of other Crustacea, and the former in that 



