MORPHOLOGY AND TAXONOMY OF THE SPOROZOA 551 



conform to the same general plan of structure throughout but differ 

 in axial relations and in sculpturing, as well as in number and time 

 of appearance. The spore capsule always consists of two valves 

 which are independently developed and come together with a med- 

 ian suture dividing the spore into right and left halves. In different 

 types the spores may be elongated in the plane of the suture or at 

 right angles to it. The polar capsules with their coiled threads 

 indicate what most authorities regard as the anterior end although 

 spores of the Myxidiidae have thread capsules at each end of the 

 elongated spore (Fig. 164, p. 325). Lateral processes, posterior spines 

 and external sculpturing of various types distinguish the different 

 genera and species and afford a means of classification. 



Order 2. Actinomyxida Stolc. 



These are Cnidosporidia about which little is known beyond the 

 process of sporulation. In its fully grown condition the entire 

 body may be interpreted as one pansporoblast which is surrounded 

 by a membrane, and which usually produces eight spores, the mem- 

 branes of which are usually triradiate and drawn out into elaborate 

 spines. Each spore has three polar capsules containing distinct 

 protrusible filaments. 



The processes leading to the formation of spores involve fertil- 

 ization phenomena of a characteristic type. They are essentially 

 similar to those of the Myxosporidia but differ in some impor- 

 tant details. A plasmodial stage appears to be absent or rep- 

 resented by a binucleate amebula only, which develops into a 

 spore. The two nuclei divide and form 4 cells, 2 of which disappear 

 with the formation of a membrane within which the other 2 cells 

 lie. Each of these divides, forming 4, 2 of which continue to divide 

 rapidly until 8 are formed, while the other 2 remain large and 

 undivided the two-celled membrane now containing 8 small and 

 2 large cells. Ultimately the two large nuclei begin to divide in 

 turn until 8 products result and 16 cells, regarded by Caullery 

 and Mesnil (1905) and by Ikeda (1912) as gametes, lie free in the 

 cyst. The two sets of gametes differ slightly in nuclear size and in 

 staining capacity and unite 2 by 2 to form 8 zygotes. The nucleus 

 of each zygote now divides until 6 small nuclei and 1 large one result, 

 the large one destined to form a mass of sporozoites. The 6 small 

 ones arrange themselves in such a manner as to form 3 shell-forming 

 cells, while 3 of them lie within and form 3 polar capsules. The 

 germ-forming cell is not enclosed by the spore-forming cells but lies 

 outside of it and peripherally in the pansporoblast. It divides 

 repeatedly until 8, 32 or many sporozoites result (Fig. 221). 



The Actinomyxida are parasites of annelids and sipunculids and 

 the spores are invariably triradiate. The anchor or star-form 



