CELLULAR DKGENERATION AND FORMATION OF PIGMENT. 1 29 



ordinary ectoderm, pigmented streaks about 40 ju x In are 

 common. These are frequently bluntly rounded at one end 

 and tapering at the other. The pigment granules are small, 

 reaching about 0'4 n in diameter ; but the pigment is not 

 always present as discrete particles. Frequently it occurs as 

 a lining to the cavities of the pigmented structure, and in 

 the endoderm there are usualh' several masses of pigment of 

 rounded or irregular shape lying in the cavities. Often in the 

 ectoderm, and especially in the thin outer sheets of ectoderm, 

 branching and very elongated streaks arise ; these stretch 

 through the epithelial cells and do not necessarily involve the 

 whole of the cytoplasm of the individual cells through which 

 the streaks pass. 



In Sertularia operculata (text-fig. 6) pigmented cham- 

 bered structures, similar to those in Thuiaria tubuli- 

 f ormis, occur in the endoderm. In the case of the ectoderm 

 the diseased condition appears to lead to hypertrophy, resulting 

 in undue multiplication of the interstitial cells, and the nuclei 

 are in abnormal condition with their staining capacity con- 

 siderably reduced. Small clusters of these interstitial cells 

 run together, vacuolation of the protoplasm occurs and oval 

 chambers are formed with the walls croAvded with large pig- 

 ment granules (figs, e-g), consisting of little dark brown or 

 black discs (diameter about I'O/u) and rodlets (length about 

 1'3 ju). The discs and rodlets mimic cocci and bacteria in an 

 extraordinary manner ; but these bodies seem undoubtedly to 

 arise from the transformation of the protoplasm of the hydroid. 



The diseased condition of pigmentation may become very 

 acute, and may lead to constriction and ultimate transverse 

 division of the coenosarc. The constriction would appear to 

 result from the hypertrophy of the interstitial cells of the 

 ectoderm. 



In Sertularella mediterranea (text-fig. 7) the pig- 

 mented oval bodies, measuring about 17/i x S fx, mostly 

 correspond to separate cells. The bodies are not black but 

 greenish-brown, and mimic unicellular algge. As a matter of 

 fact, unicellular alga3 have been described as occurring in 



VOL. 4, PART 1. 9 



