cxliv hitroduction . 



the water-vascular ring-^ and admit fluid to filter into it, through 

 their pores, either from the exterior sea-water when they are part of 

 the external skeleton (Asteroidea, Echinoidea)^ or from the peri- 

 visceral cavity when they are contained in it (Holothurioidea). 

 Other appendag-es, either of the nature of accessory reservoirs^ the 

 ' Polian vesicles/ or of a glandular character, the ' racemose ap- 

 pendag-es/ are very ordinarily found in connection with the water- 

 vascular ring. The madreporic system may consist of a sing-le, or 

 of multiple canals and tubercles. Trichocysts have been said to 

 exist in the integument of Synaptidae, but some doubt appears to 

 attach to this statement. (See Semper, Reisen nach Philippinen 

 Hft. iv. 164.) Their integument never developes any chitinous 

 structures, but is always more or less indurated by calcareous 

 deposits, and more or less roughened by spinous out-growths. 

 The calcareous deposits may be almost, or quite microseojiic ; 

 or they may form a continuously and immovably articulated 

 skeleton for the entire animal, or for its central disc ; the spinous 

 outgrowths are similarly various in size, as also in shape, and they 

 may be movably, or immovably articulated to the subjacent inte- 

 gumentary system. In any case they are normally covered with 

 an epidermal layer of sarcode, which is often richly ciliated. Where 

 the calcareous skeleton forms a continuous capsule for the animal's 

 body, the muscular system is correspondingly reduced, and in 

 Echinoidea there are no specialized muscles except for the movable 

 spines, and for the pedicellariae homologous with them, and for 

 the manducatory organs. Where the calcareous skeleton, though 

 greatly developed, is yet made up of movably articulated pieces 

 the muscular system may be moderately developed, as in Aste- 

 roidea, where the water- vascular system is locomotor in function; 

 or greatly developed, as in Crinoidea, where it is not. Where, as 

 ordinarily in Holothurioidea, the calcareous deposits, both external 

 and internal, are reduced to a minimum, the muscular system 

 attains a very high grade of evolution. There is always a large 

 perivisceral cavity in the Echinodermata, into which the sea-water 

 finds its way. The digestive tract never communicates wnth the 

 perivisceral cavity directly, and is only rarely aproctous. Though 

 it never possesses any specialized, salivary, or hepatic glands, by 

 its possession of extensive radial diverticula (Asteriae) or of lengthy 

 convolutions (Echinoidea and Holothurioidea), the digestive system 



