III. Asteroidea. IV. Ophiuroidea. 



III. Asteroidea. 



See also Fraas, supra, p 4, Mac Munn (', 2 , 4 , 5 ) , supra, p 3, Perrier ( J ), 

 supra, p 3, and Preyer, supra, p 4. 



Perrier (') describes sensory cells in the ambulacral epithelium of Asterids, 

 which are connected by fibres that traverse the supporting- membrane with multi- 

 polar nerve-cells, hitherto regarded as forming an internal epithelium lining the 

 subtentacular cavity. The collateral organ of the water-tube orChroma- 

 togen organ [Hamann; see Bericht for 1885 I p 189] is the source of the cor- 

 puscl es found in the body-cavity. The water-tube itself is in direct communica- 

 tion with the cavity of the sacciform canal around it, and thereby with the 

 schizocoel, sub ambulacral spaces, and body-cavity. 



The observations of Vogt & Yung on the anatomy of Asterids give a general 

 confirmation to the statements of Hamann [see Bericht for 1885 Ip 189]. At 

 the lower end of the water-tube its wall is pierced by small holes which place 

 it jfn communication with the tubular space containing both it and the dorsal organ. 

 The latter has no direct connection with the exterior through the madreporite, but 

 its lacunae communicate both with the water-tube and with the schizocoel spaces 

 in the integument. Its dorsal end terminates in a hollow cord which bifurcates to 

 form an anal ring, and ten canals proceed from this ring towards the genital 

 glands as described by Ludwig and Hamann. These are not excretory, but re- 

 present the sterile genital tubes in the arms of Comatulae. 



Cuenot denies the excretory function of the ovoid gland, Tiedemann's 

 bodies, and the Polian vesicles of the Asterids. They all serve to form the pig- 

 mented corpuscles of the vascular systems and body-cavity. The cells of the 

 ovoid gland become detached and are liberated from its interior by the rupture of 

 its walls. They put out numerous anastomosing pseudopodia and develope into 

 the amoeboid blood corpuscles, sometimes uniting into a plasmodium. In like 

 manner Tiedemann's bodies and the Polian vesicles give rise to the corpuscles of 

 the ambulacral system. 



Herdman mentions an example of Porania pulvillus in which the ambulacrum 

 of one ray bifurcates 3 times within a length of 1 cm. 



Fraas finds that the minute structure of the ambulacral ossicles of Asterids 

 is very constant in each genus. They are developed from the inner layer of the 

 mesoderm , whereas the tegminal plates , formed in the outer layer, vary greatly 

 in structure according to the species. He describes the characters of the ambu- 

 lacral ossicles in 8 typical genera , and indicates their importance for palaeonto- 

 logical work. Some new Jurassic Asterids are described, in one of which 

 2 arms and a portion of the disk have been broken and regenerated, while 3 pairs 

 of upper and lower marginal plates have fused together. 



Sturtz (V 2 ) points out that many palaeozoic Asterids resemble the Liassic Tro- 

 pidaster in the presence of a mediodorsal furrow on the arms, with a row of inner 

 dorsal plates on either side ; and also that several Encrinasteriae have the madre- 

 porite on the ventral side. In some forms its position rather resembles that of the 

 problematical anal pyramid in the Cystids ; Edrioaster and Agelacrinus being trans- 

 ition forms between the two groups. 



IV. Ophiuroidea. 



See also Fraas, supra, p 4, Perrier (*), supra, p 3, and Preyer, supra, 

 p4. 



d* 



