HISTOLOGY OF THE EiNDODERM. 123 



raucli smaller and more spherical (see PI. VII, fig. 5). Sometimes, liowcver, the two sets pass 

 imperceptihly into one another without any distinct boundary-line. 



In Hydra viridis, the cellular structure of the endodcrm is veiy distinct. The contents of 

 most of the cells composing it are here peculiar, and consist of a colourless protoplasm with very 

 definite green corpuscles imbedded in it, and in almost every instance with one and occasionally 

 with two clear vacuolae excavated in it.' It was the occurrence of these vacuolaj which caused 

 Ecker,- by overlooking the proper cell-wall, to adopt the erroneous view that the whole tissue of 

 Hydra was merely a mass of vacuolated protoplasm. 



Among those cells which lie most internally and form the immediate boundary of the 

 somatic cavity in Hydra viridis, are many which are destitute of green corpuscles, but contain 

 brown, irregular granules, mostly included in a secondary cell which is itself imbedded in the 

 vacuolated protoplasm of the mother-cell. 



The green corpuscles possess a very definite form— a circumstance in which they contrast 

 strongly with the brown granules. They are spherical, and present in then- interior a lighter- 

 coloured space which gives them a close resemblance to thick-walled cells. They will be again 

 referred to. (See below, p. 136.) 



Along with those cells which contain the green corpuscles, there also occur, especially in 

 badly fed Hydra, others in which the green contents are replaced by smaller spherical but 

 colom-less bodies, which are probably the green corpuscles in an undeveloped or transformed con- 

 dition. Occasionally, also, an irregular mass of brown granules may be seen in the same cell with 

 the green granules. 



The cells which thus compose the endoderm of Hydra possess but a weak union with one 

 another ; they are easily separated by a slight force, and on becoming free immediately assume 

 the spherical figure, without any trace of their having been previously united into a tissue. 



No green matter is developed in the cells of any other species of Hydra. In Hydra fusca, 

 there occur among the cells which form the boundary of the stomach cavity, many which are of an 

 elongated piriform shape, with the broad thick end projecting into the cavity, and with their thin 

 ends imbedded among the others. Within these piriform cells, secondary cells may usually be 

 detected ; these are spherical in shape, sometimes having clear colourless contents with a nucleus, 

 while in other cases they are filled with irregular brown granules, and present no evident nucleus. 

 Sometimes the piriform cells contain only free brown granules, while we may also often meet 

 with instances in which, besides the free granules, the same piriform cell will contain the clear 

 nucleated secondary cells, and the secondary cells filled with brown granules. This may be 

 regarded as the typical condition of the same parts in the other IIydroida, though it is seldom 

 so distinctly demonstrable as in Hydra. 



The walls of the somatic cavity are probably in all Hydroida, if we except Hydra, clothed 

 to a greater or less extent with vibratile cilia. These ciha are remarkably distinct among some of 

 the TuhularidiE ; while iu other cases in which their existence has not been proved by direct 

 observation, the peculiar currents visible in the fluid contents of the somatic cavity leave no doubt 

 that it is to the agency of such ciha that these currents are mainly due. Hydra would appear to 

 constitute a solitary exception in the non-ciliated condition of its somatic cavity. 



1 Allman, "On the Structure oi Hydra viridis," in 'Brit. Assoc. Reports' for 1853. 



- Ecker, " Zur Lelire vom Ban mid Leben der cuutractilen Sub.stanz der niedersteu Thiere," 

 ' Zeitsch, f. wiss. Zool.,' Bd. 1, 1849. 



