vin ECHINODERMAT AWATER VASCULAR SYSTEM 433 



The oral tube-feet (always ten in number ?) surround the mouth, and especially 

 when food is being taken in, are subject to active swinging or pulsating movements, 

 without, however, touching the food. They have been regarded as olfactory or 

 gustatory organs. They seem to be wanting in the Cidaroida and the Echinothuridce ; 

 on the other hand they occur in those Echinidcc which otherwise possess only one 

 sort of tube-feet, viz. those with sucking discs. 



The polymorphism of the ambulacral appendages is much more 

 marked in the Clypeastroida and the Spatangoida. It must first be 

 noted that the ambulacral appendages, in those apical regions of the 

 ambulacra which are known as petaloids (r/. p. 347), serve for respira- 

 tion (ambulacral gills). They seem to be peculiarly fitted for this 

 activity by the delicacy of their walls, the want of calcareous corpuscles, 

 the increase of surface obtained by branching, the possession of 

 double pores (whereas the ambulacral appendages in other parts of the 

 body have single pores) and by the size of their ampullae. 



In the Clypeastroida, besides the ambulacral gills of the petaloids, three kinds of 

 appendages have been observed : (1) the ordinary slender tube-feet, with rounded 

 terminal knobs, scattered on the test ; (2) sessile knobs, with deep sensory 



FIG. 366. Longitudinal section through an ambulacral brush of a Spatangoid (after Loven 

 and Hamann). 1, Body epithelium ; 2, supporting rod ; 3, supporting plate of the terminal disc ; 

 4, septa ; 5, canal of the water vascular system ; 6, longitudinal muscles ; 7, nerve ; 8, circular 

 muscle fibres. 



epithelium (sensory tentacles) ; (3) short, thick tube-feet with truncated ends : these 

 occur between the ordinary feet on the oral side, and are perhaps locomotory. 



Among the Spatangoida, the polymorphism of the ambulacral appendages is very 

 marked in all the divisions except in the Echinoneidcc ; it reaches its highest point 

 in the families of the Spatangidce and Apetala. 



The ambulacral gills of the four paired petaloids have been described above. 

 We note first the characteristic ambulacral brushes which occur in the Spatangoida 

 more or less near the mouth and the anus, and in the Cassiduloida on the phyllodes 

 (cf. p. 347). The terminal plate or disc of an ordinary tube-foot (Fig. 366) is here 

 extraordinarily widened, and carries a number (usually large) of club-shaped or 

 conical, solid appendages, each of which is supported by a calcareous rod. These 

 ambulacral brushes are said to play an important part in the taking in of food by 

 VOL. II ^ F 



