652 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Arbaciadse.* — In this first part of their paper on the family 

 Arhaciadce Gray, Dr. P, Martin Duncan and Mr. W. P. Sladen treat 

 of the morphology of the test in the genera Ccelopleurus and Arhacia. 

 These two forms, as is shown by a minute study of the fossil and 

 recent species, have a great similarity of structure. In all (for 

 Arhacia nigra belongs to a different genus) the compound plates of 

 the ambulacra are formed of an adoral and an aboral demi-plate with 

 a large central primary plate. In all forms the optic pores are 

 double, and the perforation is in the adoral edge of the plate, a process 

 separating the pores. In all the forms the median or vertical sutures 

 of the interradia are marked with knobs or ridges, which correspond 

 with sockets or short grooves on the opposed plate edges. This kind 

 of dowelling is even seen in the ambulacra of Arhacia and along the 

 transverse interradial sutural edges of Ccelopleurus. 



Ccdopleurus is the oldest of the two genera : there are species 

 with the peculiar ambulacra in the Eocene, Oligocene, and Miocene. 

 The recent species from the Indian Seas only differs from the Miocene 

 form in having high and not oblique interradial plates. All the 

 species of Arhacia, which are recent forms, that were examined pre- 

 sent no greater differences than can be accounted for on the theory of 

 descent. 



Histology of Asterida.f — Dr. 0. Hamann has a preliminary 

 notice on the histology of star-fishes, in which be points out that the 

 body-wall consists of an epithelium, which is followed by a layer. of 

 connective tissue, in which the calcareous structures are developed. 

 Internally to it are a layer of circular and a layer of longitudinal 

 muscles ; the latter was described by Ludwig as the lamella of sup- 

 porting substance ; in it the so-called ossicles are developed. In the 

 layer of supporting substance which lies on the muscular layers we 

 find the canal-system of the body-wall, which is invested by an 

 epithelium. Muscular bundles, passing off from the circular layer, 

 traverse the lumen of the canals, and end or branch at definite points 

 in the layer of supporting substance. The muscular layers may be 

 traced to the ossicles; and it is pointed out that the discovery of 

 these muscles enables us to explain the movement of a star-fish and its 

 arms. The ambulacral gills are to be regarded as evaginations of the 

 dorsal wall, and have the same structure as it ; their protrusion and 

 retraction is to be explained by their possession of a similar system 

 of muscles. 



In addition to the well-known oral nerve-ring the author was able 

 to detect a nerve-plexus in the oral disk ; this consists of nerve-fibrils 

 with scattered ganglionic cells, which pass into the epithelial cells of 

 the disk. It is pointed out that we have here an arrangement which 

 is comparable to that which Dr. Hamann has already described as 

 obtaining in Holothurians. In the dorsal integument there are 

 nerve-trunks which are ordinarily set at right angles to the long axis 

 of the arm. The dorsal epithelium consists of simple supporting 



* Journ. Linn. Soc. Lond. (ZooL), xix. (1885) pp. 25-57 (2 pis.), 

 t Nachr. K. Gesell. Wiss. Gottingen, 1884, pp. 385-6. 



