xii THE VOYAGE OF H.M.S. CHALLENGER. 
become obliterated, so that an obvious cellular structure becomes obscured, or entirely 
effaced. 
The muscular fibrilhe of the ectoderm are in most cases well developed. Where 
they exist in the trophosome they always run longitudinally, and form a continuous 
fibrillated layer in contact with the external surface of the mesosarc. This ectodermal 
musculature is well developed in the body and tentacles of the hydranth. In those 
genera in which the body of the hydranth is much elongated so as to assume the form 
of a naked stem (Hy dr actinia, Clava, Clavatella, Gemmelaria, Myriothela), this part 
of the animal manifests a high degree of contractility, and the fibrillated tissue is here 
always especially well developed. In the ccenosarc of Tubularia in which this part is 
closely invested by the perisarc no fibril! se can be detected, while in the allied genus 
Corymorpha, whose stem is not as in Tubularia enclosed in a thick perisarcal tube, the 
fibrillated tissue may be traced through the whole length of the stem. It is worthy of 
remark that though no fibrillse can be detected in the coenosarc of the adult Tubularia 
these are present on the whole body of the Actinula or larval stage of this genus. The 
fibrillated tissue, however, is not necessarily absent from such coenosarcs as are enclosed 
in a firm perisarc. In Plumularia echinulata of Weismann this observer has detected 
and described the fibrillse of the ccenosarc. 
The tentacles of the hydranth in the various genera have an especially well 
developed system of ectodermal fibrillse. In the tentacles of Tubularia indivisa the 
fibrillse of the ectoderm may be seen to be true muscle-cells, being greatly elongated 
fusiform cells, each with a nucleolated nucleus . 1 A similar structure has been shown 
by Weismann 2 in the fibrillated tissue of the coenosarc of Plumularia. In Myriothela, 
however, the fibrillse would appear to be on a higher grade of development, for here 
they do not present the condition of nucleated fusiform cells. In this remarkable 
genus the fibrillse have a uniform thickness throughout, showing no tendency to thin 
away into the terminal points of fusiform cells, and are without any visible nucleus . 3 
The ectodermal fibrillse in the body of Hydra were examined by Kleinenberg 4 and 
shown by him to be in direct continuation with certain tail-like processes which are 
given off from the deep side of the most superficial cells of the ectoderm in this genus. 
Kleinenberg, believing that in this relation we have a low stage of development of a 
combined muscular and nervous system, designates the whole cell with its caudate 
process and fibrilliform continuation by the name of “neuro-muscle-cell.” This capital 
discovery of the caudate processes of the ectoderm cells in Hydra, and their connection 
with the muscular fibrillse, has been amply confirmed by subsequent observers and 
extended to many genera besides Hydra, so that it must now be accepted as representing 
1 Gynmoblastic Hydroids, p. 206, pi. xxiii. fig. 6. 
2 August Weismann, Die Entstehung der Sexualzellen bei den Hydromedusen, Jena, 1883. 
3 Allman, On the structure and development of Myriothela, Phil. Trans., vol. clxv. part ii. 
4 Nicolaus Kleinenberg, Hydra, eine anatomisch-entwickelungsgeschichtliche Untersuchung, Leipzig, 1872. 
