Dissected to show the heart and the origins of the chief vessels in situ.

The cardiac region of the omostegite and the adjoining region of the cephalostegite have been removed, as well as the tergal regions of the two first abdominal somites. The walls of an arterial pericardial sinus which surrounds the heart have also been removed, and that organ with the origins of the principal vessels has been consequently exposed. The heart is hexagonal in outline and compressed. Five arteries spring from its anterior border. One in the middle line supplies the eyes and first antennae. Two others, one on each side this median artery, pass obliquely forward to the second antennae. And finally, two hepatic arteries, not seen here, spring one from each outer and inferior angle and pass downwards between the anterior testicular lobes and the intestine to the liver. One of them is shown in Prep. 33. A single dilated trunk takes origin from the posterior border of the heart. It divides at once into two branches. One of these, the superior abdominal artery, lies immediately above the intestine in the median dorsal line. A slip of blue paper has been placed under it. The other, the sternal artery, seen in Prep. 33, passes between the commissures uniting the third and fourth thoracic ganglia.

It then divides, and one division passes forwards, the other backwards beneath the nerve-cord as the inferior abdominal artery. The muscular walls of the heart are pierced by six main valved inlets, which permit blood to enter but not to pass back. Two of these inlets are on the dorsal surface and are visible in this preparation about the middle region. Two other inlets are ventral, and the remaining two are placed, one on the right, the other on the left side. In addition to these main inlets, Dezso has described others of small size, four pairs on the dorsal and two pairs on the ventral surface. Six elastic alae cordis connect the heart to the non-muscular pericardium. Their main function is probably to antagonise the contractions of the heart itself. They may also serve to suspend it in the sinus, but the arteries are probably, in this as in other animals, the chief means by which it is kept in position.

The polygonal aspect of the Decapod heart and the presence of 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, etc, 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. 112), 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 of many other animals as well. Tetronerythin is a pigment commonly distributed in the animal kingdom and is found in various tissues, e.g. in the integument and muscles. It has been supposed by Merejkowski to have a respiratory function but this is doubtful. The yellowish-red granules, seen sometimes in the blood-corpuscles of Decapod Crustacea, are perhaps formed of it.

Heart.Bela Dezso, Z. A. i. 1878.

Circulatory System. Astacus, Krohn, Isis, 1834. Stomatopoda, Schizopoda and Decapoda. Claus, Arb. Zool. Inst. Wien, v. 1884.

Structure of vessels, etc. Haeckel, Muller's Archiv (Archiv f. Anat. und Phys.), 1857

Blood of Decapod Crustacea.Halliburton, Journal of Physiology, vi. 1885. Haemocyanin and Tetronerythrin, ibid. On the latter see also Merejkowski, Bull. Soc. Zool. France, viii. Haemoglobin in Crustacea, see lists in Halliburton, op. cit.; also van Beneden on Lernanthropus, Clavella, and Congericola. Bull. Acad. Roy. Sc. Belgique (2), 49, 1880; Id. Z. A. iii. 1880. Myohaematin. MacMunn, P. R. S. xxxix. 1885.