\ Q Q Echino dermata . 



Tiedemann's bodies are racemose glands connected with the water-vascular ring 

 and lined by cubical cells. They contain masses of large pigmented cells like those 

 of the Chromatogen organ, and probably excretory in nature. The genital 

 glands first appear as small vesicles filled with cells derived from the peritoneal 

 epithelium and surrounded by a schizocoel space. They subsequently acquire a 

 connection with the lacunar systems arising from the anal ring. These are con- 

 tained in septa traversing the schizocoel spaces as on the ventral side. The ova 

 have a follicle-epithelium and a zona pellucida with radiate striae. The enterocoel 

 and water-vascular system contain amoeboid cells which are detached epithelium 

 cells. Those found in the schizocoel, connective tissue, and blood lacunae are mo- 

 dified connective tissue cells. The muscular system is partly of epithelial origin, 

 e. g. the longitudinal fibres of the body-wall, while others, like the circular fibres, 

 are mesenchymatous. 



IV. Ophiuroidea. 

 See Metschnikoff, supra, pi 85, Nachtrieb, supra, p 185, and Hoyle, infra, p 193. 



V. Echinoidea. 



See also Metschnikoff, supra, p 185, Nachtrieb, supra, p 185. 



Duncan & Sladen (S 3 , 4 ) give a detailed account of the varying composition of 

 the ambulacra and the characters of the radial plates in 4 fossil and 1 recent 

 species of Coelopleurus , and in various species of Arbacia. In all of them, except 

 Arbacia nigra, the compound plates of the ambulacra are formed of an adoral and 

 an aboral demi-plate with a large central primary plate. There is always a double 

 optic perforation in the adoral edges of the radial plates, and the median sutural 

 unions of the interradii consist of an imperfect knob and socket arrangement which 

 may extend into the transverse sutures of Coelopleurus, and into the ambulacra of 

 Arbacia. 



Duncan (') finds that in Diadema the first and third parts of the compound am- 

 bulacral plates never become demi-plates, but remain primaries ; and he gives a 

 minute description of the sutures between the various coronal plates. The ambu- 

 lacral plates of Echinothrix are arranged as in Diadema. So are the triplets of 

 Astropyga, but they are here and there made into demi-plates, and the two sets 

 are united in one great plate. \\iMicropyga there are two demi-plates, their paired 

 pores alternating with those of the central primary. 



Duncan ( 2 ) extending his researches to the ambulacra of the regular fossil 

 Echinoidea, finds that they present six well defined types of structure, viz. theCi- 

 daroid, Diadematoid (see above), Arbacioid (see above), Echinoid, Cyphosomoid, 

 and Diplopodous ; and notes the value of the ambulacral characters for systematic 

 purposes. 



Duncan ( 3 ) gives the name Perignathic Girdle to the composite structure 

 which gives attachment to the muscles of the jaws in an Echinoid. It consists of 

 ambulacral processes (auricles) which are usually united into arches, and of 

 ridges which connect the processes on the side remote from the ambulacra. The 

 Cidaridae have no processes ; but their so called auricles are homologous with the 

 ridges of the Glyphostomata which have true ambulacral processes as well. The 

 relations of these structures to the ambulacral and interradial plates in the different 

 genera are described in detail. Among the Clypeastridae there are ambulacral pro- 

 cesses but no ridges in Clypeaster, and interradial ridges only in Laganum, Mellila 

 and other genera. 



