10 Echinoderma a. 



Fewkes describes the development of the Pluteus in Ophiopholis aculeata. 

 The yolk has a transparent cortical layer which envelopes it till the 8-cell stage. 

 The blastosphere is invaginated to form the gastrula which becomes flattened on 

 the future ventral side. The mesoderm cells originate in 2 lateral clusters at the 

 pole of invagination which is strongly pigmented , especially at the sides of the 

 mouth. 



Kbhler ( 2 ) criticises Apostolides' description of the madreporic gland in 

 Ophiurids [see Bericht for 1882 I p 188]. He finds it to consist of radiating tra- 

 beculae of connective tissue which interlace at the periphery and form a very 

 narrow meshwork. They support cells of clear protoplasm which are much less 

 numerous at the periphery, where there is also much pigment. The blood-vessels 

 have an analogous structure, their lumen being reduced by threads which support 

 cells and pigment masses. There is no aboral blood- vascular ring as described by 

 Ludwig [see Bericht for 1880 I p 261], who was misled by the presence of small 

 muscular bundles in the interbrachial spaces. 



Levinsen supplements Lyman's description of the genital pouches of the 

 Astrophytidae , regarding them as enormously enlarged bursae , rather than as 

 belonging to the body cavity [see Bericht for 1881 I p 204], 



Davy describes a new Ophiuran (Protaster daoulasensis) from the Lower Devo- 

 nian of France, which shows no trace of any plates covering the arms. 



Sturtz ( ! , 2 ) notes that the madreporite is situated dorsally in the palaeozoic 

 family Protophiureae ; and he regards the ventral arm - skeleton as constituting 

 a double row of superambulacral plates (under arm plates) , and not as the am- 

 bulacral plates which they have been hitherto supposed to be. 



V. Echinoidea. 



See also Fraas, supra, p 4, Mac Munn i 1 , 2 , 4 , 5 ) , supra, p 3, Barrels, supra, 

 p 5, and Perrier ( J ), supra, p 3. 



Fewkes has effected artificial fertilization in Echinarachnius parma. No 

 polar globules were observed. Cleavage is regular and the resulting blastula is 

 truncated and pigmented at one pole where invagination subsequently occurs. 

 The bilateral arrangement of the mesoderm cells is less marked than in Ophio- 

 pholis ; but the first 2 limestone rods of the pluteus are formed in the mesoderm 

 on either side of the blastopore. The formation of the young Urchin from its 

 pluteus takes place as in other Echinids. The first traces of the test are trifid 

 spicules the ends of which bifurcate so as to form a complex structure which is 

 enclosed in a transparent cyst. Spines appear very early and are relatively larger 

 than in the adult. 



Prouho (*) describes the external nervous plexus of Echinus acutus. It is 

 beneath the epithelium and forms a ring round the base of each spine , where 

 nerve-cells are especially abundant. Beneath it is a connective tissue layer with 

 several multipolar corpuscles. The ambulacral nerve divides into 2 parts beneath 

 each ocular plate. One passes out through the pore to join the external plexus ; 

 the other joins its fellows to form a ring which sends fibres to the genital glands. 



Prouho ( 2 ) describes the oral blood-vascular r i n g of Dorocidaris as con- 

 nected 1) with the internal marginal vessel of the intestine, 2) with a plexus on 

 the ovoid gland which is continued into the mesenteric fold round the rectum 

 that unites the genital glands, 3) with 5 pharyngeal vessels which pass outwards 

 on to the ambulacra. The 2 oral rings (water-vascular and blood-vascular) are 

 in intimate relation by means of numerous ramifications and constitute the Polian 

 ring. The duct of the ovoid gland opens externally through the madreporite, 



