BRITISH ZOOPHYTES. 141 



six secondary and twelve tertiary septa. After the 

 tertiaiy septa have been formed, only two of each 

 succeeding group appear in each sextant, so that the 

 arrangement of a sextant containing septa up to the 

 fifth order would be as follows — 



14 3 5 2 5 3 4 1 



The septa are often perforated by foramina or sto- 

 mata, which serve to promote the circulation of fluid 

 along the chambers of the body cavity. 



The craspeda or mesenteric filaments {c7-) lie upon 

 the edges of the septa and are formed by the supporting 

 lamella splitting into two parts which enclose the 

 ovary, and then uniting in the form of a cylindrical 

 cord like "the hem of a flounced garment." The 

 craspeda consist of a middle portion containing large 

 numbers of urticating cells and two lateral laminge of 

 ciliated cells. The use of the craspeda is somewhat 

 uncertain, some observers regarding them as oviducts. 



The acontia [ac) are extremely interesting portions of 

 structure. They are present in all the Sagartidce, and 

 have the appearance of long, thread-like filaments or 

 cords, capable of being ejected through the mouth or 

 body wall to a considerable distance and then re- 

 tracted. They lie coiled at the lower end of the septa. 

 Many species eject these acontia in large numbers on 

 the least provocation. If a small portion be cut off 

 and examined under a high power of the microscope, 

 an extraordinary structure is manifest. The whole 

 acontium is seen to be a ribbon with a band of connec- 

 tive tissue, nerves, and muscular fibres, and an immense 

 number of large thread cells. This ribbon possesses 

 great powers of movement, and portions may be seen. 



