WYVILLE THOMSON^ ON ASTKRACANTHION VIOLACEUS. 107 



amount of versatility of function, corresponding to that of the 

 vascular system of Avliich it is a part, and to a great extent 

 dependent upon the peculiar vital properties of the substance 

 entering into its structure. The provisional assimilative 

 appendages are foi'med upon a type essentially lower than 

 that of the permanent digestive system of the Echinoderms, 

 and to understand their relations fully, I believe we must 

 keep in view the mechanism of nutrition in the classes inferior 

 to the Echinoderms in the zoological scale. 



In the sub-kingdom Protozoa, the entire body consists 

 solely of '^ sarcode/^ a gelatinous substance which^ though 

 apparently structureless, possesses active vital properties. 

 " Sarcode'^ alone, then, without the differentiationof any special 

 tissue or organ, may perform effectively the functions of 

 assimilation, respiration, and of voluntary motion j and con- 

 sequently a layer of this substance investing the germ of a 

 higher organism, or an appendage formed of it, might answer 

 the same purpose^ though, perhaps, in a lower degree, as if 

 the germ were provided with special provisional organs for 

 the performance of these functions. 



I shall confine my remarks at present entirely to the 

 Echinodermata ; though I believe they Avould apply v.'itli 

 equal justice to some other invertebrate groups. 



In the Echinoderms the eggs are extremely small, pro- 

 vided with only a thin layer of albumen — frequently with 

 none at all. Segmentation of the yelk is complete; the 

 whole substance being reduced to a smooth granular mass, 

 which is moulded into the form of the embryo, or of the 

 embryo with its temporary nutrient appendages. A consi- 

 derable time elapses before the differentiation in the embryo 

 of the permanent assimilative tract ; yet during this period 

 the embryo increases rapidly in size, frequently attaining 

 ten or twenty times its original dimensions. In many cases 

 this increase seems to take place by simple absorption of 

 organic matter through the entire external surface of the 

 embryo; but, in order to the temporary performance of this 

 function with such unusual activity, the structure of the 

 surface undergoes a remarkable modification. The entire 

 . embryo is invested with a thin layer of " sarcode.''^ 



In some cases the sarcode merely forms a thin continuous 

 layer, ciliated either generally or in bauds or patclies, over 

 the entire surface of the embryo. In others, as in the case 

 of Pteraster mUitaris (M. and T.) and of the Crinoids, the 

 ciliated layer investing the embryo is locomotive and respi- 

 ratory ; while at one point a ciliated oral aperture opens into 

 a short digestive tube, passing through the substance of the 



