554 THE ANIMAL KINGDOM. 



Pharynx of unknown Holothurian with calcareous plates, Moseley, Q. J. M. 

 xxiv. 1884. Cotton Spinner (Holothuria nigra) ejecting Cuvierian organs , Bell, 

 P. Z. S. 1884. 



Haemoglobin in Holothurian ( Thyonella gemmata], Howell, Studies Biol. Lab., 

 Johns Hopkins Univ., iii. (6), 1886. 



Ciliated funnels of Apneumona, Semper, 'Reisen im Archipel der Philippinen,' 

 i. 1868, p. 32. 



ECHINOZOA. 



Echinodermata in which the calcareous skeleton attains great per- 

 fection and bears spines fixed or free, whilst the muscular system is much 

 reduced. The apical system, always present in development, may be 

 obscured by the formation of other plates and is always relatively small. 

 Oral system either absent or represented by five oral plates. The oral 

 surface is ventral in locomotion. 



CLASS ECHINOIDEA. 



Echinozoa with spherical, heart-shaped or shield-shaped bodies, 

 times much flattened dor so-ventr ally. TJie plates of the calcareous skeleton 

 are well developed and usually immoveably joined edge to edge, and carry 

 move able spines. The tube feet have terminal discs supported by calcareous 

 plates, but are often much modified. The mouth lies in a peristome, and is 

 usually central but may be displaced in the anterior radius. The amis lies 

 either within the apical system (endocyclic) or outside it in the posterior 

 interradius (exocyclic). 



The outer surface of the body is covered by a ciliated ectoderm and 

 connective tissue layer which contains amoeboid pigment cells and a 

 nerve-plexus. The apical system is always well developed. It consists 

 typically of the usual plates a dorso-central, surrounded by five basals 

 and five radials. Under-basals and primary single interradials are never 

 present. Among Desmosticha the dorso-central persists as a single plate 

 only in some Saleniidae. It is usually replaced by or broken up into a 

 series of irregular plates among which the anus opens. In Clypeastroidea 

 and Petalosticha it is not traceable in the adult at any rate \ The basals 

 are large and are pierced each by one genital pore, or in Palaeo-echinoidea 

 by 3-5 pores. One the posterior is generally imperforate in Clypeas- 

 troidea and always in Petalosticha, and it appears to be absent (? aborted) 

 in the majority of the latter group. The radials are largest in the old 

 types such as Cidaridae and Saleniidae, smaller in the Petalosticha, and 

 smallest in the Clypeastroidea, the most recent group of Echinoidea. They 



1 It is figured in Echinobrissus sciitatus. (Zittel's Palaeontologie, Abth. I , Palaeozoologie, i. Fig. 

 388 d. p. 528). 



