ECHINODERMA GENERAL DESCRIPTION 23 



through all the tissues, including the skeletal, and containing re- 

 fringent granules, proteids, fat, and a yellow pigment called "echino- 

 chrome" (MacMunn, 1885) ; they seem to be specialised as bearers 

 of reserve food, as calcigenous cells (p. 28), as phagocytes, and 

 as bearers to the exterior of waste products often pigmented 

 (Durham, 1891, St. Hilaire, 1897); (b) red corpuscles with haemo- 

 globin (Foettinger, 1880), non-nucleate, but vacuolate or granular 

 in water-vessels of Ophim-oids (Fig. XIX. 3), nucleate in various 

 coelomic cavities of Holothurians (Howell, 1886; Cuenot, 1891, 

 Fig. XIX. 5); the respiratory nature of these is demonstrated by 

 their containing haemoglobin. 



Whatever may be the homologies of the hydrocoel, there is, 

 physiologically speaking, no nephridial or other excretory system 

 in Echinoderma. The function is probably performed by the 

 wandering cells just mentioned. ^^ 



The Axial Organ has had many functions ascribed to it, as 

 shown by its various names : Heart (Tiedemann), Pseudo-heart, 

 Central Blood-plexus (Ludwig), Glandular or Chromatogen organ 

 (Hamann), Lymph Gland, Madreporic Gland (Koehler), Collateral 

 or Plastidogen organ (Perrier), Ovoid Gland (Perrier, Cu6not, and 

 others), Genital stolon, Plexiform Gland or Dorsal organ (P. H. 

 Carpenter), Kidney (P. and F. Sarasin). The Sarasins (1888) 

 give a good account of the literature ; later notes of value are by 

 Cuenot (1891) and Durham (1891). Generally it is a brownish, 

 finely lobed, often pear-shaped body, showing under low magnification 

 a complicated arrangement of tissue strands (Fig. XX. 1). It does 

 not occur in Holothurioidea. In the other classes it is developed 

 in the axial sinus by irregular growth of endothelium, which forms 

 canal-like strands separated from one another by spaces derived 

 from the axial sinus ; these latter are therefore primitively 

 connected with the water -vessels and madreporite through the 

 stone canal. Strands growing out at an early age from the central 

 plexus become the gonads, but the connection may be lost in later 

 life. In association with the genital strands are also radiating 

 " haemal strands," not true blood-vessels, but serving for the 

 transmission of nutrient cells. Such cells, as well as pigmented, 

 excretory amoebocytes, are found in quantity in and about the 

 axial organ. In Pelmatozoa the axial organ, surrounded by the 

 axial sinus and the lobes of the chambered organ, stretches right 

 down the stem (Fig. XX. 2). The position of the axial sinus with 

 regard to the gut suggests that nutrient fluid passes by osmosis into 

 the axial organ, which thus serves as a kind of distributor, but there 

 is no evidence of pulsatile pump-action, i.e. it is no heart. The 

 evidence of new cell-formation is too slight to warrant the idea 

 that the axial organ is a factory of amoebocytes. There is still room 

 for study of this peculiar body, especially through experiment. 



