AND DEVELOPMENT OE MYEIOTHELA. 
567 
The arm is very contractile, and, when in different states of contraction, the cells of 
the ectoderm may often be seen forming irregular projections of various length and 
thickness. These vary from time to time in shape and size, and look so exactly like 
pseudopodial processes that without careful observation they might easily be mistaken 
for them. They are, however, mainly the result of the contraction of the arm. When 
the arm is shortened by the action of its contractile elements, the hyaline lamella is 
thrown into irregular corrugations, and these are communicated to the superjacent 
cellular ectoderm. In macerated sections of the arm the cellular ectoderm will become 
disintegrated and broken down, and then the exposed hyaline lamella will often show 
nearly an exact repetition of the pseudopodia-like projections. I am, however, inclined 
to think that, after the contraction of the fibrillated layer has thus crumpled the hyaline 
lamella and overlying ectoderm, the protoplasm of the latter exerts a certain contractility 
which exaggerates the prominence of its projections, and thus to a certain extent brings 
them within the category of pseudopodia. 
In the ectoderm of the body of the actinula we find not only the deep clavate tissue, 
but the more superficial layer of cells well developed. Here, during certain states of 
contraction, pseudopodia-like projections are also formed; and I believe that these are 
referable to the same cause here as in the ectoderm of the tentacles. 
The proximal extremity of the actinula body is capable, as already said, of acting as 
a sucker of attachment ; and here the ectoderm has acquired a considerable increase of 
thickness (Plate 56. fig. 15). The increased thickness is mainly owing to the great 
development of the clavate tissue at this spot. This tissue forms here a hemispherical 
cap over the cul-de-sac of the gastric cavity, and the elements composing it are scarcely 
at all united to one another into ramified groups. Its peculiar development here is 
probably connected with a special irritability with which this part of the walls would 
appear to be endowed. Over this cap the superficial ectodermal layer is continued, 
forming a zone of small, spherical, membranous cells with minutely granular contents. 
In the uninjured state a fine longitudinal striation may be witnessed in this part of the 
actinula (Plate 58. fig. 6, b) ; it is caused by the appearance of the terminal mass of 
clavate tissue as seen through the overlying layer*. 
The endoderm of the stem-like proximal portion of the actinula (Plate 56. fig. 15) 
closely resembles that of the transitory arms. It is composed of an external layer of 
large, clear, polygonal cells, with an internal one of small round cells filled with brown 
corpuscles. 
GENERAL REMARKS. 
I believe we are justified in regarding the claspers as true zooids rather than as mere 
organs ; and if so Myriothela may be compared with Hydractinia in the extent to which 
the polymorphism of the zooids is carried. We have here hydranths, blastostyles, 
* A very similar appearance may be seen in tbe actinula of Tubularia, and I have now little hesitation in 
referring it to a similar cause. 
4 F 2 
