HYDROIDEA. 331 



that in the case of some of the cnidoblasts in Physalia it is transversely striated 

 and the protoplasm of the cell itself contains a network of striated filaments. 

 Jickeli (M. J. viii. 1883, p. 393) appears to be of an opinion that the cnidoblasts 

 possess muscular processes. Von Lendenfeld points out, in the paper from which 

 this figure is taken, that the basal processes of cnidoblasts in Medusae and Anthozoa 

 are granular ; that in Crambessa mosaica he has observed the discharge of nemato- 

 cysts contained in cnidoblasts within the jelly when acetic acid was applied to the 

 olfactory epithelium ; that in arenicolous Actiniae contact of sand will not cause 

 discharge of the nematocysts, but that contact of the prey does so at once. These 

 facts point to some nervous control. The discharge of the thread is therefore 

 brought about through pressure exercised on the sac of the nematocyst, by con- 

 traction of the cell body called out in response (i) to direct mechanical or chemical 

 irritation of the cnidocil, or (2) to a nervous impulse generated by the will of 

 the animal set in motion by special stimuli. 



Von Lendenfeld has pointed out in the same paper the existence in certain 

 Guard polypes of adhesive cells. These cells form a globule, which approaches and 

 finally projects beyond the surface, and retains any body which comes into contact 

 with it. As such cells occur in the same polyp side by side with cnidoblasts, or 

 replacing these structures in later stages, Von Lendenfeld believes that there is an 

 homology between the two. Adhesive cells occur to the exclusion of cnidoblasts 

 in Ctenophora, 



For the nervous system in Hydroid forms add to the works of Jickeli and 

 Von Lendenfeld referred to, Chun on Siphonophora, Z. A. vi. 1883. 



For cnidoblasts and nematocysts see also Chun, Z. A. vi. 1883, and Hamann, 

 J. Z. xv. 1882. 



FIG. 9. Two nematocysts, A. partially discharged, B. undischarged from a Millepora ; after 

 Moseley, Ph. Tr. 1877, PI. ii. Fig. 2 a and b, a is slightly reduced in this copy. 



THE nematocysts are removed from the cells or cnidoblasts in which 

 they were developed, and where they usually remain until discharged and 

 torn away by the struggles of the prey. The cyst consists of two membranes, 

 an outer thick and an inner thin, both of chitinoid character. The thick 

 membrane apparently does not close over the pole at which the filament is 

 emitted. The thin one, on the contrary, forms a closed sac. The filament 

 is developed as a hollow ingrowth of one pole of this sac, and as growth 

 proceeds and the filament becomes longer than the greater diameter of 

 the sac, it is disposed in coils shown in B. When the filament is discharged 

 the discharge takes place by a process of eversion, the inner surface of the 

 hollow ingrowth becoming the outer surface of the hollow filament. This 

 fact can be readily made out in a filament, the discharge of which has been 

 arrested. The sac itself contains a homogeneous fluid, and it is apparently 

 the pressure of the surrounding cell protoplasm on the sac with its fluid 

 contents that causes the eversion of the filament. But after discharge the 

 sac is said to retain its form and dimensions unchanged. The base of the 

 filament as seen in A is dilated, and hence the clear rod-like space passing 



