20G ANATOMY OF SPECIAL FORMS. 



wedge-shaped in section, and are arranged at ratlier regular intervals in a zone concentric with the 

 circumference of the stem. They are richly ciliated on their sides with very distinct, long, actively 

 vibrating cilia. They are of unequal size, one of them especially being in almost every instance 

 considerably larger than any of the others. They are simple lacuncB, and represent the transverse 

 section of a system of longitudinal canals which are excavated in the peripheral portion of the 

 endodenn, and which take the place of the axial somatic cavity of other hydroids. Their presence 

 is indicated even to the naked eye by longitudinal bands, which are visible through the trans- 

 parent perisarc in the uninjured condition of the stem. 



By following these canals through the stem they will be seen to communicate occasionally 

 with one another by short transverse channels, while towards the summit of the stem they 

 coalesce into a common chamber which occupies the axis. 



Surrounding the peripheral portion of the endoderm is the zone of ectoderm {b). It lies in 

 immediate contact with it, there being here no intervening zone of fibrillated tissue. In the 

 ectoderm no distinctly cellular structure can be demonstrated ; only some granules and nucleus- 

 like bodies may be seen scattered through a common plasma in which numerous thread-cells 

 are also imbedded — a structure, however, which indicates a true cellular composition at an 

 earlier period of development. 



On the outside of the ectoderm and in close contact with it is the chitinous perisarc («), 

 showing no structure beyond that of concentric laminae of deposition. 



When a living and uninjured specimen of Tubularia indivisa is examined under a moderate 

 magnifying power active currents may often be witnessed in the channels of the endoderm. 

 These take a course sometimes from the proximal towards the distal end of the stem, sometimes 

 from the distal towards the proximal, while two currents may often be seen running in opposite 

 directions in two diflferent channels at the same time. They are plainly under the influence of 

 the vibratile cilia which clothe the walls of the canals. 



The Hydranth. — The hydranth (PI. XX, fig. 2) is flask-shaped, with two circlets of tentacles, 

 a proximal and a distal. The tentacles of the distal circlet surround the base of a conical hypo- 

 stome ; they are much smaller than those of the proximal ; they slightly alternate with one another 

 and appear to consist of two closely approximated verticils. Their ectoderm (PI. XXIII, fig. 4) is 

 loaded with thread-cells, but no distinct cellular structure can be detected in it. It is separated 

 from the endoderm by an intervening fibrillated layer. The endoderm consists of large cells, 

 each of which stretches transversely across the tentacle, leaving no pervious canal in the axis, 

 and conferring on the tentacle the characteristic septate condition of this part. These endodermal 

 cells contain a clear colourless fluid and present along the line of the axis accumulations of pro- 

 toplasm with coloured granules. The tentacle is eminently contractile. 



The tentacles of the proximal set are much larger and thicker than those of the distal, and 

 their contractility is much less marked. They are disposed in a single continuous verticil. 

 Their ectoderm is similar to that of the distal tentacles, but their endoderm, instead of consisting 

 of large transversely arranged cells, is formed of smaller cells rendered polygonal by mutual 

 pressure, and presenting the condition of a pith-like tissue by which the tentacle is filled to the 

 complete exclusion of all trace of an axile tube. These cells contain a clear colourless fluid, with 

 protoplasm enveloping some more opaque corpuscles and accumulated in a layer on the cell-wall. 



Between the ectoderm and endoderm of the tentacle there exists a fibrillated tissue 

 (PI. XXIII, fig. 5). I have succeeded in isolating this tissue and its component fibrillar in the 



