58 MR. T. H. STEWART ANATOMY OF ECHINODERMS. [Feb. 12, 



cellaria, and, very curiously, along the border of a spine that 1 have 

 mounted in longitudinal section for microscopic observation. These 

 spicula are much like those in some Sponges, except that in Echi7ius 

 they are formed of carbonate of lime, whilst those of the Sponges 

 are of silica. 



In Biadema they take a somewhat triradiate form (PL X. A. fig. 5). 



In Comatula also there are detached spicula in the internal mem- 

 brane of the perisome, which, when mounted in Canada balsam, form 

 beautiful polarizing objects. 



The higher forms of Echinoderms, such as the HolothuriadcB, have 

 their skeleton formed entirely of scattered spicula, except the oral 

 ring ; in each species the spicula are of a peculiar form : and all 

 microscopists know the wonderful anchor-shaped spicula and plates 

 in Synapta and the curious wheels of Myriotrochus and Chirodota. 



EXPLANATION OF THE PLATES. 



PLATE X. 



Fig. 1. A separated segment of the splanchnic skeleton of Echinus sphcera. 



A. A single segment of an alveolus, showing the surface opposed to its 



fellow half. 

 . The symphysis, b. The oesophageal, dentated border, c. The supe- 

 rior border for the attachment of the epiphyses, d. The styloid 

 process for the support of the tooth, e. The apex. 



B. The con-esponding segment ; the serrated surface opposed to the ad- 



joining pair of alveoli displayed. 



C. The external surface of an alveolar segment. 



D. The epiphyses of the alveoli. 



E. The falx ; upper and lower surfaces. 



F. The radius. 



G. G'. The tooth, front and side view. 



Fig. 2. A separated segment of the splanchnic skeleton of Biadema. 



A. An alveolar segment, the surface opposed to its fellow half. 



a. The symphysis, below the well-marked groove for the tooth. I. The 

 styloid process, c. The long slender haniular process, d. The oeso- 

 phageal border, with long, slender, dentated processes. 



B. The epiphysis of the alveolus. D. The radius. 



C. The falx. E. The tooth. 



Fig. 3. A separate segment of the splanchnic skeleton of Acrocladia Mgonaria. 



A. An alveolar segment, the surface opposed to its alveolar half. 



a. The styloid process, b. The oesophageal border, b'. Dentation at 

 the apex of this border. 



B. The epiphysis of the alveolus, a. Styloid process. 



C. The falx. ' D. The radius. E. The tooth. 



PLATE X.A. 



Fig. 1. A dissection of the splanchnic skeleton and muscles attached of Echinus 



sphcera. 



a. The protractores. b. The retractores. c. The radiates, d. The in- 

 terradiales. e. The auricular arch. 

 Fig. 2. A segment of the splanchnic skeleton of Echinus sphcera, showing the m- 



teralveolar muscle (a). 

 Fig. 3. A transverse section of the tooth of Cidaris, in outline. 

 Fig. 4. A similar section of the tooth of Echinus. 

 Fig. 5. Spicula from the cirri of Diadema. 

 Fig. 6. Spicula from the cirri of Echinus. 



