INTRODUCTION. 39 



<'. If. ^liiijihiiifd xijiiaiiKihi . The young of the recent viviparous 0{)liiuroidea spend 

 their early stages in the genital bursse. These are five pairs of large sac-like 

 invaginatious which penetrate into the coelom of the disc (Text-fig. ol). They 

 comraunicate with the exterior l)y means of slit-like apertures, which lie at the 

 base of the arms on the lower (oral) side of the disc (Text-fig. 12). The 

 openings are strengthened on their radial side by the " genital plates " and on the 

 inter-radial side by ossicles called the " genital scales." Gonads are attached to 

 the walls of the bursse. The main function of the liursee is respiratory. They 

 are lined with cilia, Avhich luring about a constant inward current of fresh sea- 

 water, the oxygen in which diffuses through the thin wall of the sac into the 

 coeloraic fluid (MacBride, ■i'i, p. 48o). Occasionally, however (as mentioned 

 above), they act as brood pouches. 



If we accept the view that the Aspidosomatidae possessed genital bursa? we 

 have a ready explanation of the fact noticed by Schondorf, that the marginalia of 

 the Aspidosomatidse are distinctly separate from the adambulacralia of the arm 

 ((34, p. 50). The marginalia near the arm (PL I, fig. 8) are arranged in a way 

 that at once suggests that they strengthen the boundary of a slit at the edge of 

 the disc corresponding to the genital slit in Recent Ophiuroidea. The structure, 

 together with the presence of the young forms in the disc of Axpidosoma grayse, 

 shows the length to which the homoplastic correspondence in race development of 

 the true Ophiuroidea and the Aspidosomatidge has proceeded (see p. 52). 



SUMMARY OF LITERATURE. 



Our knowledge of the structure and relationships of Palaeozoic Asterozoa may 

 roughly be grouped round three periods : 



(1) The period of the work of Forbes and Salter in England and Billings and 

 Hall in America, 1845 — 1867. 



(2) The period of the Avork of Stiirtz in Germany and Gregory in England, 

 1885—1899. 



(3) The recent researches of Schondorf and Prof, and Miss Sollas. 



The First Pekiod. 



A summary of the knowledge in 1863 fi-oni the point of view of the English 

 palaeontologist is given by Wright (81, pp. 22 — 37). Almost the whole of the fossil 

 forms knoAvn at that time had come from comparatively few localities and horizons. 

 The specimens described by Forbes were collected in the Bala (Ordovician) rocks 

 of Wales, the Ludlow (Silurian) rocks of AVestmoreland and the Wenlock Lime- 

 stone (Silurian) rocks of Dudley, Staffordshire. Salter describes numerous species 

 found only in the well-known Leintwardine (Lower Ludlow) rocks on the l^orders 



