126 PHYLUM ECHINODEBMATA. 



Otocysts are found in Holothurians (Synaptidae and Elasi- 

 poda), in connexion with the radial nerves (p. 256). 



Finally the sphaeridia of Echinoids (p. 227) are probably organs 

 of some special sense. They are modified spines, and are richly 

 provided with nervous tissue. It has been suggested that they 

 are organs of orientation, enabling their possessor to perceive 

 its position in space. 



The muscular system is very variously developed in the different 

 classes. In Holothurians, only, is there a well-marked dermo- 

 muscular body-wall. In all other Echinoderms the skin is not 

 contractile or but slightly contractile, and the muscles are 

 restricted to special bands acting on particular skeletal plates or 

 other part of the body, and to the muscular elements in the 

 walls of the tube-feet and water- vascular system generally and 

 of the viscera. In the spiniferous forms there are special muscles 

 attached to the base of the spines and pedicellariae. 



The muscular tissue consists of smooth contractile fibres with- 

 out transverse striation, except in the case of the muscles of some 

 of the pedicellariae and of a few spines. 



The Coelom. Our knowledge of the coelom of Echinodermata 

 has been of slow growth. It began with the discovery of A. 

 Agassiz * that the enteron of the larva of Asterias berylinus gave 

 rise not only to the alimentary canal of the adult, but also to the 

 body-cavity and water-vascular system. This discovery was 

 established and extended by the work of Metschnikoff on 

 Asteroids, Echinoids, and Ophiuroids, of Kowalevsky and 

 Selenka on Holothurians, of Bury on Crinoids, and of Ludwig 

 on Asteroids. Finally the recent work of MacBride has not only 

 elucidated the relations of the genital organs and of the so-called 

 perihaemal spaces, but has thrown light upon the nature of that 

 organ which was formerly called the " heart " but is now more 

 commonly referred to as the axial organ or ovoid gland. 



The coelom arises from the enteron of the embryo as a single 

 pouch. This pouch soon separates from the enteron and divides 

 into a number of sacs, one or one pair of which constitutes the 

 hydroeoel, the others the splanehnoeoel. The left hydrocoelf 

 gives rise to the water- vascular system, while the splanehnoeoel 



* " Embryology of the Starfish," Contributions to the Natural History 

 of the United States, 5, 1864. 



f The right hydroeoel, when present, is always very small, and takes 

 no part in forming the water- vascular system. 



