FREE-SWIMMING SPOROSACS OF THE HYDROID GENUS DICORYNE. 261 



close contact with the perisarc, especially in the distal region. At the level of the 

 distal edge of the hydrotheca the cells of the ectoderm are generally more columnar, 

 elsewhere they are less regular. Among the ectoderm cells of the stem may be 

 noticed a few large nematocysts. The bases of the ectoderm cells are prolonged into 

 very thin contractile processes which run longitudinally. The ectoderm is separated 

 from the endoderm by a well-defined lamella of mesoglcea. The endoderm of the 

 stem is a layer of flattened epithelial cells with granular protoplasm and large 

 vesicular nuclei in most of which a large nucleolus is present. Here and there are 

 large oval cells crowded with deeply staining spherules, probably of excretory 

 nature. 



In the greater part of the hydranth the mesoglceal lamella is more strongly 

 developed than in the stem, but diminishes in thickness towards the mouth and also 

 on entering the tentacles. The ectoderm at the apex of the hypostome contains 

 numerous oval nematocysts. The largest of these are about 6 /a long and 3 fi broad, 

 but many others are only about two-thirds this size. The endoderm, which in the 

 stem is thin, becomes in the hydranth a very thick layer. Among the ordinary 

 elongate, vacuolated endoderm cells are many clavate cells packed with spherules of 

 secretion. The wide lumen of the hydranth narrows rapidly proximally, i.e. on 

 approaching the stem, to a canal about 25-30 /x, in width which is continuous through 

 the similar lumen in the stolon with the cavities of the neighbouring individuals 

 of the colony. 



The tentacles are solid, the endoderm of each being composed of a single column 

 of discoidal cells with stout walls. Each cell is much vacuolated, the protoplasm 

 being aggregated into a small mass around the central nucleus, from which a few 

 strands radiate. The nematocysts of the tentacles are small and oval, about 4 [x long 

 and 2 /a broad. 



Blastostyles (PI. VI). 



The blastostyles arise either directly from the stolon or from the hydrocaulus, 

 but most of them are borne on the stolon. Those arising from polyps (fig. 1, B v) 

 are more recently formed than the majority of those on the stolon ; possibly they 

 are formed in seasonal succession, first on the stolon and later on the hydrocaulus. 

 A blastostyle arising from a hydrocaulus is at first difficult to distinguish from a 

 polyp-bud of the same size, but the latter soon develops tentacles and a mouth, 

 structures which are not present in blastostyles. The basal portion of each blasto- 

 style is protected by an envelope of perisarc the distal region of which is sometimes 

 moderately thick, widened into a shallow cup-shape, and easily seen ; but more often 

 the perisarc is thin and almost transparent. Invariably, however, the distal margin 

 of the perisarc is marked by a coating of extraneous matter. 



The blastostyles vary in form according to their phases of development, and there 

 are also considerable individual variations. Young blastostyles are, for the most 

 part, nearly cylindrical or slightly dilated at their distal ends, where numerous 



