80 MARGARET 0. COLLCUTT. 



"itself by its colletoderm " (which is, according to Strethill- 

 Wrightj an epidermis replacing the corallum at the growing 

 extremities of the branches), " and secretes the horny plate of 

 " the corallum. On this plate, by a further process of secretion 

 "from the lower ectoderm, the grooved spines are erected. 

 " That the upper layer of ectoderm is naked over the greater 

 " part of its surface, or only covered by a thin epidermis ; but 

 " occasionally this layer also takes its share in the secretion of 

 " the corallum, and in that event produces the smooth conical 

 "spines, the concavity of which it fills." 



Strethill-Wright conveys the impression that the endoderm 

 of the polypary is a continuous layer, permeated by tubular 

 excavations, which are connected with the digestive cavities 

 of the polyps. 



With respect to the edge of the colony he says, '' On the 

 less exposed parts of the shell the polypary frequently passes 

 beyond the papillary corallum as a thin membranous expan- 

 sion, or breaks up into a loose network of delicate anasto- 

 mosing tubes Propagative stolons are given oflp by 



these tubes ; a delicate chitinous investment may 



also be detected on the creeping tubular fibres, from which the 

 stolons of Hydractinia take their rise; but I have not 

 satisfied myself as to its presence on the entire upper surface 

 of adult polyparies." 



Two kinds of nematocyst, differing in size, are mentioned 

 by Strethill-Wright, who is the first observer of the spiral and 

 tentacular polyps. He describes the spiral polyps as mouth- 

 less, with rudimentary tentacles and a highly developed 

 muscular coat, large nematocysts being crowded in the ecto- 

 derm of the tentacles and the whole body. The tentacular 

 polyps he regards as always present. " Their tips are covered 

 with a dense pavement of the larger thread-cells, and a few 

 of the same bodies are thinly scattered along their whole 

 length." 



Van Beneden (11), in 1866, published another paper on 

 Hydractinia, in which he says that the tentacles are gener- 

 ally absent in the reproductive polyps. What he formerly 



