92 THE VOYAGE OF H.M.S. CHALLENGER. 
CHARACTERS OF THE SUB-ORDER HyYDROCORALLINE, AND OF THE FAMILIES AND 
JENERA CONTAINED IN IT, MODIFIED SO AS TO REPRESENT THE PRESENT KNOWLEDGE 
ON THE SUBJECT. 
Sub-order HyprocoraLLin&, Moseley." 
Compound Hydroid stocks, growing by gemmation. Hydrophyton consisting of 
a meshwork of ramified ccenosarcal canals, composed of an ectoderm and pigmented 
endoderm, lodged within channels permeating a hard calcareous support, ‘‘ ecenosteum,” 
which is deposited by the ectodermal investment of the canals, and forms masses of 
very various shape. Surface of the Hydrophyton covered with a continuous layer of 
ectoderm. Zooids of two forms—the one provided with a mouth and gastric cavity, 
“ oastrozooid” ; the other mouthless and simply tentacular in function, “ dactylozooid.” 
Tentacles, when present, mostly with knobbed extremities. A well-defined muscular 
layer present in the zooids. Zooids lodged within chambers, ‘“ gastropores” and 
“ dactylopores,” excavated in the substance of the Hydrophyton, lined by reflections of 
the surface layer of the ectoderm, forming the “sacs” of the zooids. Zooids of the 
two forms either scattered irregularly over the surface of the stock, or gathered into 
groups more or less regular, in each of which a centrally-placed gastrozooid is 
surrounded by a ring of dactylozooids. Cavities of zooids communicating with the 
coenosarcal meshwork by large canal offsets. 
1. Family Mitteprorip#&, L. Agassiz. 
Coenosteum irregular in growth, arborescent or encrusting, composed of a thin super- 
ficial living layer, supported by a dead mass made up of successive preceding dead 
layers. Pores devoid of styles, divided into a series of vertically succeeding chambers 
by transverse calcareous partitions, “tabule”; usually scattered irregularly, but in 
some species grouped with tolerable regularity into systems, in which a centrally- 
placed gastropore is surrounded by a ring of dactylopores. Nematocysts of two kinds 
present—the one, the three-spined form, occurring only in Hydroids ; the other ovoid 
in shape, with a thread beset with a spiral of spines. Gastrozooids short, cylindrical, 
with from four to six tentacles with knob-like tips, set in a single whorl. Dactylo- 
zooids long, filiform, and tapering, with an irregular number of short knob-bearing 
tentacles set on at irregular intervals. Gonophores unknown, but not contained 
within special cavities in the substance of the ccenosteum “ ampullee.” 
? Prelim. Report, Phil. Trans. Roy. Soe., vol. clxvii., part 1, 1877, p. 132. 
