22 Echinoderma. 



the same age (30 days) bore a single azygos arm, like that of the Spatangid 

 Pluteus. 



The cousins Sarasin ( l , 2 , 3 ) give a detailed account of some points in the ana- 

 tomy of Asthenosoma urens. The primary ambulacral plates of the corona gene- 

 rally remain isolated and do not fuse into larger plates. At the apical pole they 

 show embryonic features, and slight changes in their arrangement would give the 

 characters of Melonites, Palaeechinus, Archaeocidaris , Lepidocentrus . etc. The 

 auricles rest partly on the ambulacral and partly on the interambulacral plates. 

 Owing to the absence of interambulacral plates on the buccal membrane a 

 triangular space is left inside the ring of gills, which is covered with delicate 

 overlapping plates of secondary formation. The periproct is also covered by con- 

 centric rings of small plates which are enclosed by a ring of 5 ocular and 5 genital 

 plates isolated, however, from one another. The latter are not pierced by the 

 genital pores which open independently in the perisome within rings of small 

 plates ; while the terminal tentacles are sometimes not completely surrounded by 

 the ocular plates. - - The peculiar movements of the test, described by W. 

 Thomson and Agassiz, are due to the presence of 5 pairs of longitudinal mus- 

 cles. They are broad semilnnar sheets, each consisting of numerous bundles 

 which join the outer ends of the ambulacral plates, sometimes one and sometimes 

 many being attached to one plate. The upper bundles are inserted into the ocular 

 plates and the lower into the aboral edges of the auricles, all together joining a 

 central tendon which occupies the middle of the inner edge of each sheet. Indi- 

 vidual bundles often exchange fibres, thus producing a complex network. Smaller 

 groups of fibres arising from the adoral faces of the auricles are inserted into the 

 angular ambulacral plates on the buccal membrane. The fibres are brown and 

 smooth, and grouped a few together round the periphery of a small cavity en- 

 closed by connective tissue which is derived from the general sheath of the whole 

 bundle. Towards the central tendon they become paler, spread out and interlace, 

 so as to form a strong membrane, while their other ends are thickened and 

 attached to the coronal plates by connective-tissue fibres. The paired muscle 

 sheets serve as mesenteries to the first coil of gut which hangs in ascending 

 and descending loops attached to the 2 sheets of each pair, and is supported by a 

 connective- tissue mesentery across each interambulacrum. The second coil of 

 gut has a connective-tissue mesentery of the usual character. This internal mus- 

 culature seems to occur in other Echinothuridae, and was doubtless present in the 

 Palaeechinidae ; while the looped course of the gut and the arrangement of the 

 mesenteries in Diadema are the remains of a similar arrangement in the other 

 regular Urchins. The musculature has the same relation to the ambulacral nerves 

 and vessels as the longitudinal muscles of the Holothurians, and the pharyngeal 

 retractors attached to the radial pieces of the calcareous ring in the latter group are 

 represented in Asthenosoma by the 5 pairs of retractors of the lantern which are 

 attached to the adoral faces of the auricles. - - The extensive space between the 

 2 muscles of each ambulacral pair is occupied by one of the large Stewart's 

 organs which reach 5 cm in length. They are internal diverticula of the jaw- 

 chamber, which are present in A. in addition to the external gills, but have no 

 respiratory function, as supposed by Prouho. They are simpler than those of the 

 Cidaridae, having no supplemental diverticula nor spicules in their walls. Traces 

 of them occur in Diadema and even in Toxopneustes ; and their presence in these 

 forms must be inherited, owing to the low position of the Echinothuridae among 

 the other Urchins. The so called ovoid gland or axial organ, which is really 

 a kidney, has a spiral form like the stone-canal, its lower end resting on the 

 double vascular ring above the lantern. Its central canal is here blind, then 



