ECHINODERMA. 115 



from its attached to its free extremity. The sides of this groove are 

 perforated by alternating rows of minute foramina, and external to 

 these are situated the largest and most numerous spines. 



The tubular feet or tentacles are protruded through the marginal* 

 pores of the furrows, which are termed Ambulacra. These feet have 

 muscular parietes, and they communicate with internal vesicles, full of 

 fluid, which form, in fact, the bases of the feet. By the contraction of 

 the parietes of the vesicle the fluid is injected into the tentacle, and 

 protrudes and extends it : when the muscular parietes of the tentacle 

 contract, the fluid is returned into the sac, and the tentacle is 

 shortened and retracted. The basal vesicles are in communication 

 with, and are supplied by, a system of tubes and larger pendent 

 pyriform sacculi, which are lodged in the central disc or body of the 

 star-fish, and surround the oral aperture. 



There are other kinds of soft contractile appendages to the integu- 

 ment, some tufted, others of simple form ; but the tentacula just 

 described are the most important organs for prehension and locomotion. 

 The tegumentary processes called " pedicellariae," which resemble 

 miniature pincers, will be more particularly described in connection 

 with the skeleton of the Echinus. 



There are certain species of star-fish called Ophiurce, in which the 

 rays are extremely attenuated and elongated, and have neither am- 

 bulacral grooves nor tentacula. Nor is this complicated mechanism 

 here needed, for the flexile and spinous rays can twine around and seize 

 other objects so as to perform directly the oflaces of prehension and 

 locomotion. The facility with which the Ophiura casts off a ray 

 which may be touched and even all the rays, leaving only its central 

 disc, when it is seized, is very surprising ; it is consequently very 

 difficult to preserve specimens of this genus entire. To do this it is 

 recommended to plunge them suddenly into fresh water when they 

 instantly die in a state of the most rigid extension. I may state that 

 the Ophiura is one of the most ancient forms of animal life that 

 has yet been met with in the fossiliferous strata of our climate. 

 Professor Sedgwick has lately discovered it in one of the oldest 

 members of the Silurian system of rocks. 



The mouth of the star-fish (Asterias) is situated at the middle of 

 the under surface of its body ; it is edentulous, and leads by a short 

 gullet into a large stomach, (Jiff. 64?. a), which sends off" a pair 

 of sacculated csecal appendages (6 Z») into each of the i-ays. The small 

 terminal pouches of these appendages appear to secrete a substance 

 subservient to chylification ; two or more small glandular sacs (c c) of a 

 yellowish colour open into the bottom of the stomach, and have been 

 regarded as a rudimental form of liver. 



I 2 



