290 ZOOLOGY FOR MEDICAL STUDENTS chap, viii 



cells scattered through the ectoderm and particularly abundant on the 

 tube-feetj all of which may be said to function as simple sensory organs. 



The Echinoderms are typically radially symmetrical, and as radial 

 symmetry is a characteristic of animals that are sedentary or at least not 

 in the habit of moving actively in one particular direction we may assume 

 with considerable probability that the various Echinoderms as we now 

 know them have evolved out of ancestors sedentary in habit as is the 

 case still with the great majority of Crinoids. In studying the develop- 

 ment of the Echinoderms we find however that the great majority pass 

 through a larval stage in which they swim freely and, in correlation with 

 this, show a temporary bilateral symmetry. 



These Echinoderm larvae while showing certain resemblances to the 

 trochosphere of Annelids exhibit conspicuous peculiarities of their own. 

 The most striking of these is the localization of the cilia on the surface of 

 the body into a band which undergoes a great increase in length either 

 by assuming a tortuous course over the surface of the body as in the 

 Holothurian larva shown in Fig. 119, B and C, or by being carried out 

 on slender prolongations of the body as in the Ophiuroid larva shown in 

 Fig. 119, A. The precise modifications differ in the different subdivisions 

 of the phylum, the end result being a set of larval forms of very charac- 

 teristic shape and, especially when seen alive, of great beauty. Such 

 Echinoderm larvae exist in great swarms during summer and autumn in 

 quiet inlets of the sea such as the sea-lochs of Western Scotland, and 

 they are easily collected by dragging slowly through the surface waters 

 a tow-net of fine muslin or silk-gauze of the kind used for sifting flour. 



BOOK FOR FURTHER STUDY 

 Sedgwick. A Student's Text-Book of Zoology, Vol. III. 



