The question as to whether or not there is a blood system in the Starfish has an interesting history. It must be remembered that the examination of the structure of Echinodermata was first undertaken by human anatomists, who approached the subject imbued with the idea that representatives of all the systems of organs found in the human subject would be found in the lower animals also. So the perihaemal canals were originally described as blood-vessels. Later, Ludwig[[449]] discovered a strand of strongly staining material running in each septum which separates the two perihaemal canals of the arm. Each of these radial strands could be traced into connexion with a circular strand interposed between the outer and the inner perihaemal ring-canals. This circular strand again came into connexion with a brown, lobed organ, lying in the wall of the axial sinus, and this in turn joined at its upper end a circular cord of pigmented material adhering to the dorsal wall of the coelom (lying in fact within the aboral sinus), from which branches could be traced to the generative organs. Ludwig concluded that he had at last discovered the true blood-vessels, though the facts that the radial strands and the oral circular strand absorbed neutral carmine strongly and that the vertical and aboral strands were pigmented, constituted a very slender basis on which to found such a conclusion. The colour apparently appealed to the imagination, and it is undoubtedly true that the "plasma" or blood-fluid of other animals often absorbs stain strongly.
The strands were accordingly named "radial blood-vessels," "oral blood-ring," "aboral blood-ring"; and the brown vertical strand was called the "heart," although no circulation or pulsations had ever been observed. When later investigations revealed the fact that the so-called heart was practically solid, the term "central blood-plexus" was substituted for heart, although it was still regarded as the central organ of the system. The name "perihaemal" was given to the spaces so called because they surrounded the supposed blood-vessels.
In order to come to a satisfactory conclusion on the matter some general idea as to the fundamental nature and function of the blood-vessels in general must be arrived at. Investigations made on various groups of animals, such as Annelida, Mollusca, Crustacea, Vertebrata, show that at an early period of development a considerable space intervenes between the alimentary canal and the ectoderm, which is filled with a more or less fluid jelly. Into this cavity, the so-called "primary body-cavity" or "archicoel," amoebocytes, budded from the ectoderm or endoderm or both, penetrate. In this jelly with its contained amoebocytes is to be found the common rudiment both of the connective tissue and of the blood system. The resemblance of the archicoele and its contents to the jelly of a Medusa is too obvious to require special insistence on, and therefore in the Coelenterata it may be stated that there is to be found a tissue which is neither blood system nor connective tissue but is the forerunner of both.
In the higher animals as development proceeds the jelly undergoes differentiation, for some of the amoebocytes become stationary and connected with their pseudopodia so as to form a protoplasmic network. A portion of this network becomes altered into tough fibres, but a portion of each strand remains living, and in this way the connective tissue is formed. In the interstices of the network of fibres a semi-fluid substance (the unaltered jelly) is found, and this is traversed by free, wandering amoebocytes. In other places the jelly becomes more fluid and forms the plasma, or liquid of the blood, whilst the amoebocytes form the blood corpuscles. The blood system thus arises from regions of the archicoel where fibres are not precipitated.
Now in the Starfish the whole substance of the body-wall intervening between the ectoderm and the coelomic epithelium really represents the archicoel. The formation of fibres has, it is true, proceeded to a certain extent, since there are interlacing bundles of these, but there are left wide meshes in which amoebocytes can still move freely. Apart from the skeleton, therefore, the tissues of the body-wall of the Starfish do not exhibit much advance on those of a Jellyfish. If anything is to be compared to the blood system of the higher animals it must be these meshes in the connective tissue. From observations made on other Echinoderms it appears probable that the colour of the skin is due to amoebocytes loaded with pigment wandering outwards through the jelly of the body-wall and disintegrating there. The strands regarded as blood-vessels by Ludwig are specially modified tracts of connective tissue in which fibres are sparse, and in which there are large quantities of amoebocytes and in which the "jelly" stains easily. Cuénot[[450]] suggests that they are placed where new amoebocytes are formed; this is quite possible, and in this case they ought to be compared to the spleen and other lymphatic organs of Vertebrates, and not to the blood-vessels.[[451]]
The organ regarded as the heart, however, belongs to a different category: it is really the original seat of the genital cells and should be termed the "genital stolon." Careful sections show that at its upper end it is continuous with a strand of primitive germ-cells which lies inside the so-called aboral blood-vessel, and is termed the "genital rachis" (Fig. 192, g.r). The germ-cells are distinguished by their large nuclei and their granular protoplasm. The genital organs are only local swellings of the genital rachis, and from the shape of some of the germ-cells it is regarded as highly probable that the primitive germ-cells wander along the rachis and accumulate in the genital organs. The genital rachis itself is an outgrowth from the genital stolon, and this latter originates as a pocket-like ingrowth of the coelom into the wall separating it from the axial sinus; when fully formed it projects into and is apparently contained in this latter space.
Not all the cells forming the genital stolon become sexual cells. Many degenerate and become pigment-cells, a circumstance to which the organ owes its brown colour. In very many species of Starfish many of the cells of the genital rachis undergo a similar degeneration, and hence is produced the apparent aboral blood-vessel. Further, the rachis is embedded in connective tissue which has undergone what we may call the "lymphatic" modification, and this for want of a better name we call the "aboral" blood-ring.
The size of the genital organs varies with the season of the year; they are feather-shaped, and attached to the genital rachis by their bases, but project freely into the coelom of the arm. From their great variation in size and also from the shape of some of the cells in the genital rachis, Hamann concludes that as each period of maturity approaches fresh germ-cells are formed in the rachis and wander into the genital organ and grow there in size. It is probable that the aboral end of the genital stolon is the seat of the formation of new germ-cells.
In the Starfish, therefore, as in other animals with a well-defined coelom, the genital cells ultimately originate from the coelomic wall.
The genital ducts are formed by the burrowing outwards of the germ-cells. When it is remembered that the fundamental substance of the body-wall is semi-fluid jelly, this process will be better understood.