80 THE ECHINODERMATA. § IT. 



covering the shell has distinct muscular bands for the motions of the 

 points. <*> 



In the Holothurioi'dea and Sipunculoi'dea there is a very thick sub- 

 cutaneous muscular layer. This is itself composed of two layers, — the first 

 and upper being made up of circular, the second and lower of longitudi- 

 nal fibres. In the Holothurioi'dea,^'* these fibres form five large, thick, 

 ■widely-spread bundles, which are inserted into the osseous ring. In the 

 Sipunculoi'dea, these bundles are more numerous, but more compactly 

 bound together. ^^' 



The muscles of mastication, of the digestive canal, and of the tentacles, 

 ■will be treated hereafter. 



§ 77. 



With the exception of the Synaptinae and Sipunculoi'dea, the Echinoderm» 

 have special, tentacular, locomotive organs (ambulacra). These are hollow 

 and very contractile prolongations of the skin, and communicate througk 

 the ambulacral pores with small contractile sacs (ambulacral vesicles), found 

 upon the internal surface of the coriaceous or calcareous envelope of the 

 body. The ambulacra and their vesicles have transverse, longitudinal fibres,, 

 and contain a clear liquid, which, from contractions, oscillates from one tO' 

 the other through the pores. In this way the ambulacra are capable of 

 erection and elongation, and the animal uses them as feelers to find a 

 proper object of attachment ; and on this account, also, they have in some 

 species a suctorial extremity. 



These oi^ans, which are sometimes locomotive, sometimes prehensile, 

 have the following variations of structure and form : 



I. With the Crino'i'dea they are small, delicate and cylindrical, and are 

 found upon the borders of a furrow, which runs from the mouth along the 

 soft perisoma covering the arms and pinnulae. Each one of them is cov- 

 ered with small cylindrical, clavate tentacles. ^^> 



II. The Ophiuridae have upon their arms, and between the plates, pores 

 which connect with small cylindrical ambulacra ; these last, from numerous 

 small warts, present a studded aspect.'^' 



III. With the Asteroidae they are situated in a double or quadruple 

 row, in the ventral furrows which extend from the mouth to the end of the 

 rays. They form compact cylinders of considerable size, the acute or 

 truncated extremity of each of which has a sucker.'^* 



IV. With the Echino'idea they are situated upon an elongated stalk, 

 and have a sucker. They are found both upon the ambulacral plates and 

 immediately around the mouth. <*> Being extremely movable, they are 



< Valentiriy Monogr. loc. cit. p. 35, PI. III. fig. dae are attached to surrounding objects ; see 



59. Erdl in IViegmann's Arch. 1842, I. p. 58, Taf. 



J The cutaneous muscular system of Holothuria II. fig. 1, a. 



has l)een described by Tiedemann (loc. cit. p. 27, 3 Beside the very correct description giv«i of 



Taf. II. IV.); and that of Synaptn by (^uatre- these organs by Tiedemann (loc. cit. p. 56), see- 



fagen (Ann. d. Sc. Nat. loc. cit. p. 41). Rymer Jonen (A Oen. Outl. of the Anira. King. 



6 For the muscular system of Sipunculus nn- p. 148, fig. 65). It appears that in Astropecten 



dus, see Grube, ia MuUer^s Arch. 1837, p. 240, the e.\treraity of the ambulacra can be inverted,, 



Taf. XI. fig. 1. thus compensating for the sucker found in Echi- 



1 The ambulacra of Comatula, which have ac- naxter, Aste.riscu.i, and Asteracanthion. 



live vermicular movements, have no oi>ening at •• With Echinus the suckers, which exactly xe- 



their free extremity ; see Müller, Abhand. d. Berl. semble the other ambulacra, are fixed upon thQ 



Akad. loc. cit. p. 222, Tab. IV. tig. 13, 14. contractile membrane surrounding the mouth. 



^ By these the very active arms of the Ophiuri- With Upatansus and Ech.inanth.us there is 



