ECHIXODERMATA GENITAL ORGANS 



495 



the digestive sac and the apical body wall by means of strands of 

 connective tissue, and consist of the same layers as the body Aval), 

 though in the bursae these layers are much thinner. Calcareous 

 corpuscles may be either present or wanting in the connective tissue. 

 The inner epithelium of the bursae is in some parts strongly ciliated. 



The outer apertures of the bursae lie at the sides of the proximal 

 portions of the arms, which are included in the disc. Each bursa has, 

 as a rule, one aperture, but in 

 the genus Ophinn.i. (formerly 

 Ophioderma) there are two aper- 

 tures on each side of the base of 

 an arm, one distal and the other 

 proximal. Both these apertures, 

 however, lead into one and the 

 same bursa, and the double aper- 

 ture (in Ophiura) can be deduced 

 from the ordinary single aperture 

 by assuming that the margins of 

 the latter fuse at about 'the 

 middle of their length. 



The gonads are attached to 

 the wall of the bursa, on the 

 side turned to the body cavity 

 (Figs. 391 and 392). The 

 spinal nrorhirts nass into the of the stomach and the gonads (after Ludwig). 

 5 6XUal prc a Of the two burs*, that on the left has been removed. 



blirsa, and are ejected thence j, Dorsal shields of the ami; 2, dorsal wall of -the 



FIG. 389. Part of a preparation of Ophio- 

 glypha similar to that in Fig. 388, after removal 



disc ; 3 > bursa with its tJ P ( 4 >; 



through the aperture. This is, 



i F ,1 F 6, vertebral ossicle in the base of the arm ; 7, genital 



however, Only Olie Of the func- ptate : 8> row of tawal plates or scales. 



tions of the bursa, and, in most 



Ophiuroidea, as it appears, not the principal function. 



The bursse have a more important function as respiratory organs. 

 Sea-water can enter them, and through their thin walls exchange of 

 gases can take place between it and the body fluid. It would be in- 

 teresting if it could be proved that, as in the mouth and oesophagus 

 of the Corals, the sea-water enters through one (more or less proximal) 

 part of the bursal aperture, and flows out again through another (more 

 distal) part. The proximal aperture of each bursa in Ophiura is per- 

 haps an inhalent, and the distal an exhalent aperture. 



In certain Ophiuroidea (e.g. Amphiura squainata, magellanica, Ophia- 

 ntntha ricijuii'ii. marsupialis, Ophioglypha he.radis, Ophiomym rivipara, etc.), 

 the bursae serve as brood chambers. The eggs pass through their 

 Avhole development in them, until all the organs of the young Ophiurid 

 are formed. 



2. The Genital Apparatus (Figs. 390-393). 



The most interesting point in connection with the genital apparatus 

 is the peculiar course of the apical ring sinus with the ring-like strand 



