312 Royal Society : — 



coats and the integument, bearing a resemblance in appearance to 

 the spiracles of insects. Being placed in the interseptal spaces, they 

 have a perpendicular arrangement, but are not regularly disposed in 

 any other respect. They can be opened widely, or perfectly closed 

 at the will of the animal ; and are well seen, under a low power of 

 the microscope, when a Sagartia hellis or dianthus is much dis- 

 tended in a parallel-sided glass vessel, with a strong light behind it. 

 The width of these orifices varies from g^^th to ^V^h of an inch. 

 No ciliary current passes through them. 



Under irritation the Sagartia forcibly and repeatedly contracts its 

 body, forcing out the water which had distended its aquiferous canals 

 and the general cavity of the body. Much of the fluid finds vent at 

 these foramina, carrying with it the free floating part of some or 

 other of the numerous acontia^ each through that cinclis which 

 happens to lie nearest to it. The frequency with which the acontia 

 escape in a loop or bight, shows that the issue is the result of a 

 merely mechanical action, viz. that of the escaping water. 



The cnidce occur under four distinct forms. 1. Chambered cnidae 

 (^Cnidce cameratce). This is the most widely distributed, and the 

 most elaborately armed. In Cyathina Smithii they occur of com- 

 paratively large size, and are therefore well suited for observation. 

 They are transparent, colourless vesicles, of a long, oval figure, -j^Trth 

 of an inch in length, and 2^-oVo^^ ^^ diameter. A fusiform chamber 

 passes through the centre of the anterior moiety, merging at one 

 extremity into the walls of the cnida, and at the other diminishing to 

 a slender chord, which is irregularly coiled within the general cavity. 

 Under stimulus the cnidce suddenly expel their contents with 

 great force. In general the eye can scarcely follow the excessive 

 rapidity with which the chamber and its twining thread are shot 

 forth. When fully expelled, the thread, which I distinguish by the 

 term ecthorcBum, is often thirty times as long as the cnida ; but in 

 Sagartia generally, it frequently is not more than once and a half 

 the length of the cnida. 



In the ecthorcBum from chambered cnidce the basal portion is 

 distinctly swollen ; thence, becoming attenuated, it runs on as an 

 excessively slender wire of equal diameter. Around this basal part 

 wind one or more spiral thickened bands, varying, in different spe- 

 cies, as to their number, the number of volutions made by each, and 

 the angle which the spiral forms with the axis. The direction is 

 from east to north. The spiral armature I call the screw, or strebla. 

 There is no other form of armature than this. 



These thickened spiral bands afford insertion to a series of fine 

 setce, which I c&W pterygia. These are from eight to twelve in a 

 single volution, and they project in a diagonal direction from the 

 ecthorceum, but often become reverted. In some cases, perhaps in 

 all, the strebla and the pterygia are continued beyond the swollen 

 portion of the ecthorceum, even to the end of the attenuated part. . 



2. Tangled cnidae {Cnidce g J omiferce). This sort differs from the 

 preceding chiefly in the uniform slenderness of tlie ecthorceum, which 

 lies coiled up more or less regularly in the cnida, without any 



