262 PR J. H. ASHWORTH AND DR JAMES RITCHIE ON THE 



ncmatocysts are present (fig. 1, B i). In some cases, however, the blastostyle is more 

 clavate, and in others is even more dilated, being almost ovate distally owing to the 

 great development of the ectoderm of the region bearing the nematocysts (fig. 1, B in). 

 The nematocysts are of two kinds : those at the tip of the blastostyle are large and 

 elongate oval in form (PI. VII, fig. 7), their length being about 15-16 /x and their 

 breadth 4-5 /a ; the nematocysts on the sides of. the distal portion of the blasto- 

 style are smaller and of more rounded oval form, their length and breadth being 

 about 6-7 ft and 4 /a respectively. 



In young blastostyles the endoderm consists of high columnar cells, filled with 

 finely granular protoplasm, so that the ccelenteron is represented by a narrow axial 

 slit, but in mature blastostyles a large ccelenteron is present, though in the distal 

 portion — corresponding to the hypostome of a hydranth — the cavity is smaller. The 

 greater part of the endoderm of such a mature blastostyle is composed of elongate, 

 thin-walled cells which are highly vacuolated, but the endoderm of the hypostomal 

 region consists of smaller columnar cells similar to those of the young blastostyle, 

 and their granular protoplasm is but little vacuolated. Among the ordinary 

 endoderm-cells are globular cells — numerous in some blastostyles, more scanty in 

 others — containing masses of granules, probably excretory, which stain deeply with 

 iron-heematoxylin. 



The ectoderm of the column of the blastostyle is similar to that of a hydranth, 

 and its cells are provided with muscle-processes which run longitudinally, but at the 

 distal end of the blastostyle the ectoderm is thicker and heavily charged with 

 nematocysts of two kinds (see above). The external surface of the ectoderm is 

 covered by a thin cuticle. 



The mesoglcea of the middle region of the blastostyle attains a great development, 

 often reaching a thickness of 6 fi, and on the endodermal side is raised into many 

 irregular folds. In the hypostomal area the mesoglcea is much thinner, and at the 

 tip of the blastostyle is a film of extreme tenuity. 



Oocytes about 6 /u, in diameter are distinguishable in the ectoderm of the blasto- 

 styles by the time the latter have attained a length of little more than '2 mm. They 

 are situated in a zone around the blastostyle near the middle of its length. By the 

 time the blastostyle is about '5 mm. long the oocytes become so large as to cause 

 ectodermal elevations * — the beginnings of the sporosacs — on the surface of the 

 blastostyle. About this time the blastostyle generally becomes swollen in its middle 

 region and is henceforward vase-shaped, the sporosacs being borne on this wider 

 portion (fig. 1, B iv). The distal region, which throughout the life of the blastostyle 

 is armed with the two varieties of nematocysts described above, remains narrower 

 and somewhat dome-shaped. The longest blastostyles observed are about "7 mm. in 

 length (see fig. 1, B iv). 



* Occasionally such elevations are present in smaller blastostyles, in one case when the blastostyle was only 

 ■25 mm. long (see fig. 1, Bn), but this is exceptional. 



