672 PROF. SCHULZE ON THE STRUCTURE AND ARRANGEMENT 
below this a semicircular internal transverse enlargement, with a like thicken- 
ing also with numerous sting-capsules. The remaining portions of the arm are 
quite flat. 
The thread-cells, which, besides appearing in the ectodermal thickening of 
the arms, are also found, though isolated and not yet in a vertical position, in 
the endodermal layer of the ccenosarc, resemble in shape the broad sting- 
capsules of Hydra, although they do not attain to so great a size. 
The endoderm of the ccenosare contains flat cells, but in the hydranth bodies 
it consists of tall clear ciliated cells. As direct processes of the endoderm layer, 
a single row of columns, formed of clear cells, stretch to the terminal knobs of 
the arms. 
Fig. 1. Amphibrachium euplectelle (Schulze), 
The Hydroid in situ, infesting the soft tissues of Zuplectella. x 60. 
A simple layer of very delicate, hyaline, supporting lamellz, are found in 
the outer surface of the arms, as well as in the body of the hydranth. 
Reproductive buds could unfortunately not be observed. 
On account of the number and situation of the arms, I call this commensal 
of Euplectella aspergillum, given sixty times the natural size in the above wood- 
cut, Amphibrachium euplectelle. 
