clii Introduction. 



digestive tract ; which, in the rest of its course, is intestiniform and 

 attached by a fenestrated mesentery in festoons round the interior 

 of the shell. They are never aproctous. 



The pseudhaemal system has a circular vessel developed both at 

 the oral and apical poles, the ring at the oral pole lying inferiorly 

 to the water- vascular ring at the base of the dental apparatus. The 

 two rings are connected by a pulsatile sac, the so-called ' heart,' and 

 vessels are given off to the viscera, and especially to the intestine. 



Special respiratory organs are developed in many EcUnidae in 

 the shape of arborescent outgrowths communicating with the peri- 

 visceral, but, according to most authorities, not with the water- 

 vascular system, and arranged in the peristomial area. They are 

 absent in the Echinoidea with petaloid ambulacra, the widely 

 expanded lamellar gills carried by which exercise an aerating 

 function. 



The nerve-pentagon lies immediately below the oral peristoma, at 

 a much lower level than the pseudhaemal and water- vascular rings. 

 The nerve-stems, which are given off from it radially, are much 

 wider in the middle part of their course than at either distal or 

 proximal end. 



It is of importance to note, that, though growth ordinarily takes 

 place in Echinoidea in the way of interpolating fresh plates at the 

 apical pole of the corona, similar additions may be made in Cidaris 

 at the oral pole to the movable plates which, in that genus, are 

 prolonged on to the peristomial area from the immovably arti- 

 culated corona. 



The generative glands in the regular forms are five in number, 

 opening by five separate ducts in the five genital plates. In the 

 irregular forms, Spatangidae and Clypeastridae, there are only four 

 genital glands, ducts, and plates. 



The larva is pluteiform, and strengthened by a calcareous frame- 

 work. The embryo appropriates no part of the larva except the 

 stomach and some formative blastema which is aggregated round it. 



Professor Grube is reported by Dr. Semper, Eeisen im Archipel der 

 Philippinen, p. 163, to have met with an Echinid with a perfectly soft 

 integumentary system. The structural differences between such an animal 

 and a Holothurian would be comparatively small — the resemblances very 

 numerous. Amongst them may be mentioned, as probably existino-, the 

 possession of calcareous discs by the feet ; the presence of specially mo- 



