The development and function of the heart and pericardium in echinodermata

The investigations which form the subject of the paper were begun with the object of verifying the statements made by several authors with regard to the origin and development of the so-called "heart" or pericardial vesicle of Echinoderms. A study of the literature soon led to the conclusion that our knowledge of the development of the organ was somewhat defective and that a thorough revision of its development and ultimate fate would be desirable. This work has occupied my attention for the last two years and the results obtained have been fairly satisfactory.Historical Resumé . The Echinodermata offer a number of most interesting problems to the comparative physiologist. They are in many senses the lowest animals, from the point of view of organisation, which possess a true cœlom or secondary body-cavity. It was always assumed by earlier naturalists that these animals must have a circulatory system and strenuous efforts were made to find a heart and blood-vessels. None of these efforts has been very successful because the so-called vessels were found to be mere rents in the loose connective tissue without proper walls of their own, and further, no connection could be traced between vessels in one part and those in another part. The fact that these vessels owe the honour of being denominated blood-vessels at all is because they contain a ground-substance, which unlike the ground-substance of the rest of the connective tissue, stains with aniline dyes such as eosin and methyl green. If there is to be true circulation some part of the system must be rhythmically contractile and so a heart had to be found. A pillar-like organ, lying alongiside the stone-canal in Echinoidea, Ophiuroidea and Asteroidea, was selected for the rôle. Unfortunately, in Holothuroidea, where the so called vessels are best developed, it is absent and this fact may be correlated with the elongated shape of the animal and the contractility of the body wall. Later, as repeated observation had failed to detect any sign of its beating, the non-committal term of "pseudo-heart" was adopted for it. It is now proposed to call it the “pericardial vesicle.”