ECHINODERMA GENERAL DESCRIPTION 3 



Ophiuroid (Fig. XIII. p. 261) resembles a starfish in which there is a 

 sharp distinction between arms and disc ; the mouth is on the 

 under surface, but there is no anus. Whereas the arms of a star- 

 fish are merely extensions of the body, containing the generative 

 glands and processes from the stomach, those of an Ophiuroid con- 

 tain only blood-vessels, water -vessels, and nerves, and,, being 

 themselves used as locomotor organs, have a stout internal skeleton 

 of separate ossicles, worked on one another by well -developed 

 muscles, but have less developed podia ; they are nearly always 

 live, and unbranched except in Astrophytidae (Fig. XXXII. p. 277). 

 As is explained on p. 238, however, no sharp line can be drawn 

 between Asteroid and Ophiuroid structure. A Crinoid (Fig. III. 

 p. 98) differs markedly from all the forms just mentioned, in that 

 the mouth faces upwards, or away from the sea-floor ; the anus 

 is also on the upper surface. This position is connected with the 

 fixed habit of the Crinoids, which are attached temporarily or 

 permanently to the sea -floor by their aboral surface, usually 

 through a jointed stem. This fixed state of existence is corre- 

 lated with the development of a jointed process ("arm" or 

 brachium) from each radius of the rigid theca. The arms are 

 often forked many times ; they contain extensions of the nervous, 

 blood-vascular, water-vascular, and generative systems, and have a 

 ventral groove lined with cilia which sweep currents of water to the 

 mouth. The Blastoids (Fig. IV. p. 82) may be roughly described 

 as Crinoids without brachia, but with food-grooves on the oral 

 surface of the theca, fringed with jointed skeletal processes 

 (brachiola). The Cystidea, like the Crinoids, are fixed, with mouth 

 and anus on the upper surface ; the relations of their food-grooves 

 and water-canals vary greatly ; in some of the older ones (Fig. II. 

 p. 44) radial symmetry does not seem to have affected even these 

 organs, still less, therefore, any other organs of the body. The 

 Edrioasteroidea (Fig. VI. p. 209) are sessile, with upwardly directed 

 mouth and anus, with five food-grooves radiating from the mouth, 

 sometimes on to the aboral surface, as in Echinoids, and apparently 

 with hypothecal water-canals, also as in Echinoids, at any rate 

 with some portions of the water-vessels penetrating the test along 

 the ambulacra. 



Phylogeny and Ontogeny. The combined evidence of com- 

 parative anatomy, embryology, and palaeontology indicates that 

 the Echinoderma owe most of their obvious characters, such as 

 radiate symmetry, the ambulacra, and the coil of the gut, to their 

 having passed through a " pelmatozoic " stage, i.e. a stage in which 

 the animal was attached by a part of its body wall, in which the 

 mouth, and to a less extent the other apertures, faced upwards, 

 while there was a tendency to the radiate (pentamerous) exten- 

 sion of food-grooves with accompanying organs (see Chapter IX., 



