THE SKELETON. 25 



(q. v.) as perforate masses of calcareous matter of no determ- 

 inate figure. The remains of such skeletons form immense 

 deposits in the earth's crust. In Thallasicolla (sea jelly), the 

 large cells composing the mass are loosely connected by sili- 

 ceous spicules. In Lithocampa, the skeleton is compact, dome- 

 shaped, with the cyst contained within the dome. The 

 sarcodous mass is protruded at will through numerous fenes- 

 trse. In Infusoria, the skeleton is membranous, ciliated, has 

 the precise form of the animal, and is often furnished with 

 spine. In Ceratium, two horn-like processes are appended to 

 a reticulated shell. 



RADIATA. Ccelenterata. Polyps naked. In Alcyonium, 

 integument with granules of lime interspersed without order 

 or forming calcareous masses coral. These last are (1) fixed, 

 rigid, as in Madrepora, or segmented, with corneous tissue, 

 with or without spicules intervening, as in Isis and Gorgonia; 

 (2) root loosely inserted in the bottom of the sea, of a leathery consist- 

 ence, as in Pennatula (sea pen), or having, in addition, granules 

 of lime interspersed, as in Renitta. In Acalephse, the hydroid 

 stock of Hydroida is either coriaceous, branched, furnished 

 with hydra cups and capsules, or naked. In Millepora alone 

 is there a calcareous, coral-like skeleton. Porpita has a circu- 

 lar, radiate, calcareous disk; Velella, an upright triangular 

 crest. The Discophorse are naked. 



Echinodermata. Segmented in all ; mobile, as in Pentacri- 

 nus, Comatula, Asterophyton, Asteracanthion ; fixed as in Toxo- 

 pneustcs (sea urchin) and Spatangus; diffuse, as in Holothuria 

 and Psolus squamatus ; calcareous oesophageal ring present in 

 latter group. In Synapta, curious anchor- shaped appendages 

 to skin. Deciduous form of skeleton seen in larval form of 

 Toxopneustes. (See p. 127.) 



MOLLUSCA. Polyzoa. Skeleton compact for accommoda- 

 tion of many individuals ; each cell perforate from within to 

 accommodate protrusions of visceral sac. 



Brachiopoda. Skeleton bivalved (shell), placed to the body 

 antero-posteriorly ; opened by spiral valve. 



Tunicata rarely with -skeleton. Spicules of carbonate of 

 lime, resembling raphides of plants, not unfrequent in mantle. 



