18 Echinoderma. 



municates laterally at regular intervals with the brachial coelom , the pavement 

 epithelium of which passes , towards the sinus , into scattered nuclei irregularly 

 disposed on connective tissue fibres. In the disc this communication may be more 

 or less obscured by the plates round the genital slit (Ophiurids), or by the dispo- 

 sition of the mesentery (Euryalids) ; but there are no interradial communications 

 between the coelom and the ring-sinus as in Asterids. The ring-sinus gives 

 origin to the axial glandular sinus, containing both water-tube and ovoid 

 gland. In Ophiurids the gland is attached by a short pedicle to the connective 

 tissue covering the dorsal face of the nerve-ring , and becomes pyriform near the 

 madreporite to enclose the twisted water-tube. In Euryalids the ring-sinus is not 

 enclosed in the bony parts of the peristome as in Ophiurids, and it gives origin to 

 a large axial sinus , only partially filled by the ovoid gland ; this is pierced at its 

 base by numerous vascular cavities , lined by cubical epithelium and extending 

 into the connective tissue in the wall of the gullet. These represent the rudi- 

 ments of a vascular network on the digestive tube, as in Urchins and Holo- 

 thurians. The structure and function of the ovoid gland in Ophiurids are the 

 same as in Asterids ; but in the Euryalids it is only active for about half its 

 length. None of the pore- canals in the madreporite of Euryalids lead into 

 the glandular sinus. As in Asterids there is a genital cord or lymphatic 

 gland producing amoebocytes, which probably escape into the sinus round it 

 perihaemal canal ofLudwig). The latter extends over the genital caeca and 

 contains numerous amoebocytes , but has no continuous epithelial lining , only 

 irregular nuclei. The genital cord contains nuclei and small cells like those of 

 the ovoid gland, some surrounded with granules of the albuminogenous ferment. 

 They pass gradually into the primordial ova which increase in number towards 

 each genital caecum. The caeca are identical in structure in the young of both 

 sexes , being filled with these primordial cells. In the testis only the peripheral 

 cells (spermatospores) become functionally active , the remainder being resorbed 

 or discharged. Their nuclei give rise by budding to a number of smaller ones which 

 form an elongated group (spermosphere) with the spermatospore at the end. These 

 nuclei or spermatoblasts are attached to a slender rachis and the inner ones mature 

 first, the head of each spermatozoid detaching a refringent granule which escapes 

 into the surrounding liquid. After their liberation the spermatospore commences a 

 new cycle of development. In the young ovary the primordial cells are surrounded 

 by a network of connective tissue, the tracts of which contain yellow yolk granules 

 and each represent a vitelline rachis. The young ova are attached to the 

 rachis by pedicles through which the vitellus reaches them, and the vitelline mem- 

 brane when it developes is continuous over this on to the rachis. 



Under the name of Ophiopteron e%sLudwig describes a new Ophiurid with a 

 pair of fins on each arm-joint which must serve for swimming purposes. The side 

 arm-plates are expanded laterally and bear spines of 3 kinds: 1) hooks, each 

 with 2-4 booklets; 2) longer spines, swollen at the base, and beset distally with 

 spinelets ; 3) the 10 supporting rods of the fin, slender, tapering, and of different 

 lengths. Each fin is formed of a thin transparent membrane uniting these rods, 

 and overlaps the aboral part of the one next it, when seen from the dorsal side. 

 The 2 fins on each of the first 3 free arm-joints are united by their hinder edges ; 

 while the first 2 joints within the disc have no fins, and the third but an imper- 

 fect one. - - On the dorsal face of the disc each spine is continued into 6 diver- 

 ging rods which are united by a thin membrane to form a sort of funnel. These 

 funnels cover the disc closely and conceal most of the radial shields, becoming 

 more scattered in the mterradial spaces and disappearing ventrally. 



