A COLLECTION OF HYDEOIDS. 277 



The perisarc, terminating in a groove at the base of the 

 polyp, is generally slightly inverted into a shallow cup (figs. 

 5 and 6, p.cp.), which is doubtless comparable to the sheath 

 seen in E. vagina turn. 



The ectoderm is provided with two kinds of neniatocysts ; 

 the large measure about 2o"Oju in length, and 10'4fx in 

 breadth; the small variety 5 it and 2"2/u respectively. The 

 small neniatocysts occur chiefly on the tentacles, while the 

 large are found on the body of the hj'dranth, and more 

 especially around the edge of the reflexed ectoderm of the 

 hypostome (fig. 6, n.). 



The character of the endoderm is remarkable ; there is 

 practically no digestive cavity, and the mouth is blocked up 

 by a plug of elongated endoderm cells, continuous with the 

 digestive endoderin at the base of the polyp. 



The upper surface of the hypostome consists of endoderm 

 of two kinds — the central plug of digestive cells above 

 mentioned (fig. 6, e.p.), and the outer reflexed epithelium [r.e.) 

 continuous with the skeletal endoderm which runs up the 

 tentacles. Between the elongated endoderm cells are wedged 

 glandular cells {v.c), which stain intensely Avitli ha^matoxylin ; 

 the protoplasm is densely crowded with small, rounded 

 vacuoles. 



The plug of endoderm can directly ingest food; /./., in 

 fig. (J, represents a cluster of copepod eggs. The exposed 

 surface of endoderm is probably sticky during life, since small 

 Gopepods and other organisms may frequently be found 

 adhering to it. 



This throwing out of digestive endoderm through the 

 mouth, for the ingestion of food, is, as far as I am aware, 

 unique among the hydroids. 



Systematic Position. — The general character and branching 

 of the hydrocaulus is distinctly that of an Eudendrium. 

 The hypostome is also trumpet-shaped, although it has special 

 peculiarities. It is remarkable that these peculiarities are 

 present in the developing hydranth of Parawrightia 

 robusta IVarren ('Ann. Xat. Gov. Mus.,^ vol. i, p. 192). 



