INTRODUCTION. XXV 



another, sucked in, as it were, through an imperceptible 

 orifice. 



Acontia are less universal than craspeda, for whereas 

 the latter are always present, so far as I know, in this 

 order, the former are found only in the Sagartiada, and 

 perhaps in the Bunodidee. In Sagartia bell is they spring 

 from the mesenteries that carry the craspeda; generally 

 two acontia from each mesentery, and most frequently 

 in pairs. Their point of insertion may be anywhere in the 

 length of the mesentery, great irregularity prevailing in 

 this respect. 



Though at first it seems a solid cylinder, the acontium is 

 really a flat narrow ribbon, with involute and approximate 

 edges, which can at pleasure be brought into contact, 

 and thus constitute a tube (Plate XI. fig. 4). Like the 

 craspedum, of which it seems to be a form modified for 

 a special use, its surface is richly ciliated ; and the ciliary 

 currents not only hurl along whatever floating atoms chance 

 to approach the surface, but cause the detached fragments 

 themselves to wheel round and round, and to swim away 

 through the water. Though there is not the slightest 

 trace of fibrillae in the structure of the acontium, even under 

 a power of 800 diameters, the clear sarcode, of which 

 its basis is composed, is endowed with a very evident 

 contractility. 



Under pressure, the edges of the flattened acontium 

 appear to be thronged with clear viscous globules, over- 

 lapping one another, and protruding ; indicating one or 

 more layers of superficial cells, doubtless forming the 

 peritoneal epithelium. As the pressure is increased, these 

 ooze out as long pear-shaped drops, and immediately 

 assume a perfectly globular form, with a high refractive 

 power. Below these is packed a dense crowd of cnidce, 

 arranged transversely. 



The Cinclides. The emission of the acontia is provided 

 for by the existence of special orifices, which I term 

 Cinclides. The integument of the body, in the Sagartia, 

 is perforated by minute foramina, having a resemblance in 

 appearance to the spiracula of insects. They occur in the 

 interseptal spaces, opening a communication between these 

 and the external water. 



The appearance of the cinclides may be compared to 

 that which would be presented by the lids of the human 



