184 



CHAS. B. WILSON. 



If Salensky's figure be compared witli this one of Cere- 

 bratulus (fig. 76) it will be noticed that in his figure (45, 

 fig. 8) the plate is pulled in so far that fully one half the 

 depression is formed of aboral ectoderm. Whether this is a 

 normal condition in Linens I am unable to say, but 1 have 

 found a similar condition in Cerebratulus only when the 

 apical muscle was strongly contracted. If this were true in 

 Linens the separate fibrils of the apical muscle when con 

 tracted would naturally pull out the inner ends of the cells 

 to which they were attached, and may have produced the 

 apparent prolongation. 



The long stout cilia arise in bunches of four to six, from 

 near the centre of the outer ends of the cells (tig. 76). Near 

 the border of the plate they are finer and shorter than at 

 the centre. 



In life they are gathered into a compact bundle, looking 

 like a single large flagellum, but in preserved specimens 

 they are always separated and usually broken (cf. figs. 10 

 and 74) . 



Both surfaces of the plate are covered by a fine structure- 

 less membrane, through which cilia and muscle fibrils pass. 



There are usually in the angle between the plate and the 

 ectoderm mesenchyme cells whose function is unknown. 

 They are yellowish or sometimes dark brown in colour in the 

 living larva, and no processes can be detected coming from 

 them. It is possible that they may be of the same nature 

 as the so-called " eye-spots of annelid larvae, but this could 

 not be determined. 



The plate, therefore, is developed from ectoderm cells, 

 which are smaller in size, less transparent, bear longer and 

 more sensitive cilia, and whose nuclei are relatively larger, 

 but which are otherwise undifferentiated from the remaining 

 ectoderm. 



None of these things necessarily indicate the formation of 

 anything like nervous tissue ; and, in view of the fact that 

 the fibres connected with the plate are undoubtedly non- 

 nervous in origin, structure, and function, we must conclude 



