. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. 52 HYDROIDA II. stalked and mobile, or small, rudimentary or entirely effaced, being represented merely by holes in the periderm, at times with slightly raised, asymmetrical edges. The margin of the hydrotheca is almost always without teeth. The colonies are monopodial with terminal growth point. The gastral endo- derm of the polyp is divided into a fore stomach and a digestive stomach part, the limit between them appearing as a constriction round the body of the polyp. As thus defined, the family covers the group Eleutherop
. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. 52 HYDROIDA II. stalked and mobile, or small, rudimentary or entirely effaced, being represented merely by holes in the periderm, at times with slightly raised, asymmetrical edges. The margin of the hydrotheca is almost always without teeth. The colonies are monopodial with terminal growth point. The gastral endo- derm of the polyp is divided into a fore stomach and a digestive stomach part, the limit between them appearing as a constriction round the body of the polyp. As thus defined, the family covers the group Eleutheroplea auctt. but it also further includes such primitive forms as Kirchenpaueria, which lacks true sarcothecse. What have been taken for such are in reality merely low, as a rule somewhat asymmetrically developed margins round the aperture in the periderm through which the sarcostyle passes out. This formation cannot be altogether regarded as a parallel to the sarcotheca, which forms the fixed point of support for the sarcostyle, and is as a rule furnished with a diaphragm in Phtmulariidce. On the other hand, it must be admitted that there are certain transition forms which render the limit between Plumulariidae and Aglaopheniidce somewhat indistinct at times, as we find species with both stalked mobile, and immobile, sessile sarcotheca;; these forms must be regarded as primitive AglaopheniidcB. Such transition forms are also met with as against Haleciidcr, though it has not in this case been found advisable to unite the two families. We find, as a matter of fact, in almost every hydroid family such intermediate forms, the position of which is more or less a matter of doubt. If entirely definite boundary lines had to be drawn, it would very possibly involve the reduction of the entire group of hydroids to one or two families. But the heterogeneity would then be too great. In the same way, the Plumulariidcv, as viewed by Nutting (1900) and Stechow (1913) make a too heterogeneous g
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