III. Asteroidea. 13 



of integument isolated by section causes contraction of the papulae. This occurs 

 by means of the superficial nerve-plexus which is physiologically independent. - 

 But one kind of corpuscle occurs in all the internal fluids (coelomic, blood-vascular, 

 and water-vascular), viz. amoeboid cells with refractile pigment granules and 

 long hyaline pseudopodia, mostly uniting into plasmodia. The pigment, generally 

 yellow, is least abundant in old cells and most so in moderately young ones. It 

 is respiratory in function and may be called haemoxanthin [see supra, p 2], 

 being allied to but not identical with haemoglobin. The corpuscles are produced 

 in the lymphatic glands, viz. the ovoid gland supplying the coelom and blood- 

 vessels, with Tiedemann's bodies and the Polian vesicles for the water-vessels. 

 Rounded at first, they gradually acquire the respiratory pigment and pseudopodia, 

 and creep out of the gland to fall into the surrounding liquid. When old, the pro- 

 toplasm becomes vacuolated and breaks up, after losing its pigment. Besides 

 fixing oxygen for respiration the corpuscles are supposed to absorb the peptones 

 and other products of digestion from the coelomic fluid (as they do foreign pig- 

 ments, whether soluble or granular) , and to prevent them from escaping by osmosis 

 through the papulae or lymphatic gills together with the excretions. Tiede- 

 mann's bodies are appendages of the water-vascular ring and composed of 

 slightly ramified tubes lined by a cubical epithelium, the cells of which undergo 

 constant desquamation and become amoeboid. In like manner the inner lining of 

 the Polian vesicles consists of a fibrous network forming alveoli. These en- 

 close scattered cells which acquire yellow pigment, become amoeboid and escape 

 into the fluid contents of the vesicle. In many Asterids some of the pores of the 

 madreporite lead into the tubular axial sinus round the water- tube, a diverticulum 

 of which forms a terminal ampulla enclosing the end of the ovoid gland. The 

 madreporic sac or ampullar diverticulum of the water-tube beneath the madre- 

 porite is always single, not double nor treble ; and there are no openings at either 

 end of the water- tube by which it can communicate with the tubular sinus around 

 it, as described by Perrier, Vogt & Yung [see Bericht for 1886 Ech. p 9]. In 

 some Starfishes (Asterias, Astropecten, Luidia) 2 blood-vessels can be injected, on 

 opposite sides of each tube- foot, especially of those near the mouth. The con- 

 vexities of the plicated wall of the water-tube in Asterias are covered by a lower 

 epithelium than the concavities. The most complex water-tube occurs in Luidia, while 

 that of Astropecten goes through a developmental phase which is permanent in the 

 Asteriadae. The ambulacral vesicles are simplest in this family, bilobed in Luidia, 

 and double in Astropecten. No currents pass through the madreporite, either in- 

 wards or outwards, its function being merely a secondary one. The contractions 

 of the Polian vesicles are not, like those of the ambulacral vesicles, for the pur- 

 pose of creating a circulation in the ambulacral system ; but they are only adapted 

 for discharging into the latter the leucocytes formed in the alveoli of the Polian 

 vesicle. The oral blood- sin us (perihaemal ring-canal, Ludwig) communi- 

 cates with the coelom in each interradius by a passage which lies between the 

 abductor muscle and odontophore. The radial sinus gives off a lateral branch on 

 the adoral side of each tube- foot, and these all unite into a marginal sinus, from 

 which, as well as from the radial sinus, a vessel passes on to each tube-foot. 

 Another branch from the marginal sinus opens into the coelom of the arm between 

 each ambulacral and adambulacral plate. The radial sinus is lined by a low 

 vibratile epithelium which is the direct continuation of that lining the general 

 coelom. The vertical septum traversing it is sometimes very thin; but in the 

 Asteriadae and Astropectinidae it is tolerably thick and pierced by a large hole 

 opposite each intervertebral space, so that its dorsal end appears to be forked, 

 and the radial sinus tripartite. The septum is continued into the oral blood sinus, 



Zool. Jahresbericht. l&SS. Echinoderma. f 



