330 DESCRIPTION OF THE PLATES. 



as they generally are in a solid tentacle. At the proximal end they 

 abut against the gastric endoderm, from which they are in the first 

 instance derived. Their protoplasm is much vacuolated, and surrounds 

 the nuclei in a little mass, from which radiate towards the cell-walls 

 irregular strands. A very thin layer covers the walls internally. The 

 nuclei are disposed in a linear series in the axis of the tentacle. A 

 large fat globule lies above the nucleus in the figure. This is sometimes 

 broken up into 2-3 smaller drops. 



FIG. 8. A portion of the head of a Guard-polype from a Plumularia, showing a sense-cell, 

 supporting cell, and cnidoblast in relation with a ganglion cell ; from Von Lendenfeld, Z. W. Z. 

 xxxviii. 1883, PI. xviii. Fig. 3 (cf. A. N. H. (5) xii. 1883). 



g. Ganglion cell, such as have been detected in the ectoderm of many 

 Hydromedusae including Siphonophora, It has a nucleus, granular 

 protoplasm and outrunners. 



s'. Sense-cell. The body of the cell is slender, nucleated, and bears at its 

 outer extremity a cilium, and at its inner it is prolonged into a slender 

 filament. The body of the cell is sometimes very granular. The 

 basal process has been traced into continuity with a ganglion cell. 

 Cf. Z. W. Z. vol. cited, PI. xxix. Fig. 8. 



s. Covering or supporting cell. It bears a cilium, and its outer extremity 

 is broad. The body of the cell is stouter than in s', but the basal 

 extremity is elongated. In many instances it is broad, and rests on 

 the supporting lamina. 



M. Muscle cell. The protoplasm with nucleus is on one side, the con- 

 tractile filament (dark in the diagram) on the other. 



C. Cnidoblast containing an undischarged nematocyst. The outer 

 extremity of the cell bears on the side opposite the nucleus a 

 pointed process, sensory in nature, the cnidocil. This cnidocil has in 

 some instances been observed to have a complex structure and to 

 contain an axial simple or basally trifid filament. The cell body con- 

 tains a long oval nematocyst, granular protoplasm, and a nucleus. Its 

 basal portion is attenuated, and is in relation with processes of the 

 ganglion cell. Actual continuity of the two structures has been 

 observed by Jickeli (op. cit ante, under Hydra), and by von Lendenfeld 

 in Cyanea Anaskala (Z. W. Z. xxxvii. 1882, pp. 480, 513). This con- 

 nection would explain the voluntary control which many of these 

 animals appear to exert over the discharge of the nematocyst. It 

 does not, however, exist in all instances. 



There is some dispute over the nature of the basal process of the cnidoblast. 

 In some instances it is hyaline, and Hamann believes that it is attached to the 

 supporting lamina, and is purely a supporting structure itself. Chun has observed 



