34 



ECHINODERMATA. 



mouth. It is made up of ten pieces alternately 

 larger and smaller, and gives attachment to the 

 longitudinal muscles of the body. It is re- 

 :_iivlrd as the rudiment of a skeleton, while 

 the addition of scales or plates in the skin 

 forms in some species an approach to the more 

 perfect cutaneous skeletons of the star-fish and 

 sea-nrchin. 



2. Organs of motion. The spines of some 

 Echinodermata are employed to a certain extent 

 as organs of locomotion; they have been al- 

 ready described. The star-fish has the power 

 of slowly moving its rays; it can bend them 

 towards the dorsal or ventral surface, or ap- 

 proximate some of them while it separates 

 others more widely, and thus prepare itself for 

 creeping through narrow passages. Tiedemann 

 ascribes these motions wholly to the contractile 

 skin ; they are no doubt partly effected by that 

 tissue, but Meckel describes distinct muscles 

 passing between the calcareous plates which 

 form the floor of the rays, and we have our- 

 selves observed a distinct band of muscular 

 fibres running along the roof of each ray be- 

 tween the coriaceous skin and peritoneal mem- 

 brane, and also transverse fibres, but less 

 marked, lying between the same parts; the 

 latter are seen adhering to the external surface 

 of the peritoneal membrane when it is stript 

 off. 



The muscular system of the Holothuria is 

 much more developed. Ten longitudinal mus- 

 cles (fg. 20, s, s, s,) arise from the calcareous 

 ring in the vicinity of the mouth, and pass 

 along the body in the form of broad bands to 

 the posterior extremity ; between these and the 

 skin transverse or circular muscles (/, /,) are 

 situated; they extend over the whole internal 

 surface of the skin. 



The principal locomotive organs of Echino- 



dermata are the membranous tubes named the 

 feet. These are very numerous and are usually 

 disposed in regular rows; they contain a clear 

 fluid, which is conveyed to them by a peculiar 

 system of vessels. Each foot consists of two 

 parts, an internal and generally vesicular por- 

 tion (jig. 12, </,) placed within the body, and a 

 tubular part (c) on the outside, projecting from 

 the surface and continuous with the first through 

 an aperture in the skin or shell (fig. 23, ,f). The 

 tube is closed at the extremity and terminates 

 there in a sucker, which has usually the form 

 of a disk slightly depressed in the centre. 

 Both parts of the foot are evidently muscular, 

 the fibres of the tubular portion being disposed 

 in a circular and a longitudinal layer; the 

 cavity is lined with a transparent membrane, 

 and the tubular part moreover receives an 

 external covering from the epidermis. The 

 foot is extended by the contraction of its inter- 

 nal vesicle, which forces the fluid into the tube, 

 or when a vesicle is wanting, by the projection 

 of a fluid into the tube from a communicating 

 vessel ; the tubular part is thus distended and 

 elongated ; it retracts itself of course by its 

 muscular fibres, and when this takes place the 

 fluid is forced back again into the vesicular 

 or internal part. In progression the animal 

 extends a few of its feet in the direction in 

 which it desires to go, attaches the suckers to 

 rocks, stones, or other fixed objects immedi- 

 ately in advance, then shortening its feet it 

 draws its body in the wished-for direction. 



a. In the starfish the feet are disposed in 

 rows along the under surface of the rays, di- 

 minishing in size as they approach the extre- 

 mity (fig. 7, a, b, d}. There are usually two sim- 

 ple rows in each ray, (Jig. 23, c,) and the vesi- 

 cular part is for the most part deeply cleft into two 

 lobes (as in A. aurantiaca, fig. 22, d, d). In 



Fig. 12. 



