REPORT OX THE HYDROIDA. 
XXL 
zooids, “individuals" or “personae," having devolving on them as their role in the 
physiological division of labour among the various zooids some special office which is 
probably that of aiding the hydranths, by their amoeboid prehension of solid matter, 
in the general nutrition of the colony. 
The Ectoderm of Myriothela. — The curious Gymnoblastic genus Myriothiela has been 
already more than once referred to as possessing exceptional points of structure. The 
ectoderm especially presents so many points of interest and significance that no 
exposition of Hydroid structure would be complete without some special account of it . 1 
The ectoderm of Myriothela phryyta, the only known representative of the genus, is 
composed of two distinct strata — a superficial and a deep. The superficial stratum con- 
sists of small round cells, several in depth. These are formed of membraneless protoplasm, 
and contain throughout the greater part of the body abundance of yellowish corpuscles, 
while on the summits of the tentacles and in irregular patches on other parts of the body 
the superficial cells contain granules of a dark brownish-purple colour. In this layer 
large and small thread-cells may be seen enclosed in their generating cells or cnidocysts, 
for the most part lying near the surface. 
The deep layer of the ectoderm is formed by a very remarkable tissue to which I have 
elsewhere given the name of “ chavifbrm tissue .” 2 This is composed of cells consisting of 
a yellowish granular protoplasm, entirely destitute of membrane, and each drawn out into 
a long caudal process. An obvious nucleus may frequently be seen in them. By the 
union of their caudal processes branched groups of claviform cells are produced, and the 
common stalk of each group runs to the hyaline mesosarc, where it loses itself among the 
fibrillse, which here form a well-marked muscle layer. The whole forms a soft, pulpy, 
and somewhat glandular-looking tissue easily broken down under the compressorium. 
If we except the condition of the long transitory arms of the Actinula or free loco- 
motive stage of Myriothela, the claviform tissue does not come to the surface of the 
body. Throughout the whole of the body of the adult it forms a deep zone intervening 
between the hyaline mesosarc and the superficial layer of the ectoderm. 
In Myriothela the ectodermal musculature is well developed. It forms a well-marked 
layer of longitudinal fibrillse closely applied to the outer side of the mesosarc, from which, 
after a short maceration in water, it may be separated as a continuous plate composed of 
fibrillse which adhere to one another by their sides, forming a stratum of a single fibril 
in thickness. The fibrillse are about 12 j, Uo of an inch in diameter, soft and compressible, 
with a very finely granular structure, but otherwise apparently homogeneous. They show 
no striation, no nucleus can be detected in them, aud they admit of being traced to a 
considerable distance without showing any tendency to taper away in the manner of true 
muscle cells. 
1 For a detailed account of the structure of Myriothela, see Phil. Trans., vol. clxv. pt. ii. 
2 Structure and development of Myriothela, p. 553, pi. lvi. fig. 6, &c. 
