8 Echinoderma. 



system of tubes containing a nutritive fluid , as the seat of formation of the pig- 

 ment cells of the coelom which reach it by way of the perihaemal canals. There is 

 no connection between the water-vessels and either the body cavity, perihaemal 

 canals, or vascular system. 



Cuenot( 2 ) finds that the nerve-bands of Ophiurids have the same structure 

 as those of the Asterids, viz. longitudinal fibrils lying between the bases of high 

 epithelium cells, the nuclei of which have been described as nerve-cells by Teuscher 

 and Kbhler. The oral ring gives off 2 branches in each interradius , one to the 

 external interradial muscle, and another which sends branches to the dental pa- 

 pillae. Large branches are given off from the radial nerves which pierce the 

 brachial ossicles and supply the intervertebral muscles. Other branches, provided 

 with small ganglionic swellings proceed to the spines. The radial blood- 

 vessels described by Ludwig and Kbhler are only fibres and cells of connective 

 tissue without any morphological value [see Bericht for 1886 Ech. p 10]. Lud- 

 wig' s perihaemal canal is the only structure belonging to the vascular system. It 

 is connected with an oral ring which is joined to an aboral ring by the axial sinus 

 enclosing the water-tube and ovoid gland. The aboral ring gives off vessels which 

 surround the genital caeca, these being united by a genital cord situated within 

 the aboral ring as in the Asterids. 



Hamann ( 2 ) points out that the Ophiurids resemble the Urchins and differ from 

 the Asterids in the mesodermic position of the nervous system which lies in 

 schizocoel spaces within the connective tissue. It reaches a higher development 

 than in any other Echinoderms, as the radial trunks are segmented. At the origins 

 of the pedal nerves there is but a single layer of superficial ganglion cells , while 

 the intervening portions have many layers; they give off the costal nerves which 

 pass upwards through the vertebrae to the intercostal muscles, and also epithelial 

 nerves which ramify extensively to form an elaborate subepidermic plexus on arms 

 and disc, containing both bipolar and multipolar ganglion cells. This is more 

 developed than in any other Echinoderms, and its presence accounts for the physio- 

 logical results obtained by Preyer (see supra, p 4 and Bericht for 1886 Ech. p 4]. 

 Ophiolepis albida has a ciliated dorsal pore placing the enterocoel in communication 

 with the exterior, and so precisely comparable to the water-pores of Crinoids. H.'s 

 account of the oral schizocoel or perihaemal ring and its relations to the nervous 

 and vascular systems agrees with that given by Kbhler [supra, p 7] ; but he finds 

 another schizocoel ring part of which lies dorsally, beneath the radial shields, 

 and part in the walls of the genital bursae. It contains a genital tube like that 

 of Comatulae and Asterids but enclosed within a blood-lacunar ring, as in the 

 latter group. Kbhler, however, denies the existence of such a ring in Ophiurids. 



Duncan gives a detailed account of the mouth-frames, dental apparatus, genital 

 plates, arm-bones, and their attached muscles in Ophiothrix variabilis, and of the 

 arm-bones in Ophiocampsis pellicula. 



Fewkes describes the mouth of the bilateral larva of Amphiura as formed by 

 invagination while the larva is still enclosed in its sac and attached to the parent. 

 Before this attachment is severed the intestine is atrophied and lost. The pro- 

 visional skeleton (= spines of Pluteus-arms) is not always symmetrical, and 

 sometimes develops on one side. It does not entirely disappear till after the as- 

 sumption of the pentagonal form. The radials appear before the dorsocentral and 

 terminals, while the earliest plates formed on the actinal hemisome are the first 

 adambulacrals. The second adambulacrals bear club-shaped spines homologous 

 with those on the side plates of the arms in the adult. Ludwig's account of the 

 development of the ambulacral plates is confirmed. The ventrals originate as 

 median impaired plates , the first one to appear corresponding to the first adarn- 



