1893.] MICROSCOPICAL JOURNAL 95 



of sounds and bays of the North Atlantic coast of the United 

 States. In the air it appears like a grayish scum, but when the 

 crab is immersed in water it forms a soft gray velvety layer, com- 

 posed of the bodies of multitudes of Hydra-like members, collec- 

 tively forming a colony. The single members of the colony (see 

 fig. 17), called hydrozooids^ bear considerable resemblance to 

 Hydra, having a tubular body with manubrium and tentacles and 

 central mouth, but there are no gonads nor any lateral hydra- 

 torm buds, and the body is attached below to a sort of runner or 

 stolon^ which, by its budding, builds up the colony, the buds as 

 they form not separating from the parent stock as in Hydra. The 

 zooids and the stolon are both cellular and composed of ectoderm 

 and endoderm layers. Both are hollow, and thus all the mem- 

 bers are in open connection for purposes of circulation. The 

 zooids are of two sizes and kinds ; some larger ones (not shown 

 in the figures) do not bear any buds, and are solely the gatherers 

 of food for their own use and for the use of the colony ; other 

 (fig 17.) smaller ones are, besides being feeding zooids, capable 

 of producing lateral globular buds, called medusa-buds ox gono- 

 zooids^ because they are to contain \\\q. goiiads. The gonozooids 

 when ready to leave the hvdrozooid stock that formed them is a 

 bell shaped gelatinous body (fig iS.)* with a manubrium hang- 

 ing down inside and with the rim nearly closed by a thin mem- 

 brane, the veil or velum^ which has a circidar opening in the 

 centre. The margin of the bell is furnished with eight tentacles 

 at regular intervals, four longer and four shorter, at the base of 

 each one of which is a bright colored spot, believed to be 

 somewhat sensitive to light. The ectodermal part of wall of the 

 bell is highly contractile muscular cells, and from its shape 

 determines that its vigorous contractions shall force a jet or 

 water out through opening in the centre of the velum, the re- 

 action of which propels the bell in the opposite direction. The 

 gonozooids, or, as they are more commonly called, tnedusce^ as 

 they develop, free themselves from the parent body by these con- 

 tractions of the bell and then swim independently through the 

 water by the same aid and cease to have any physical relation to 

 the colony which produced them. The manubrium is, like that 

 of Hydra, pierced in the centre by a mouth surrounded by tentacles 

 and hollow within. At the base of the manubrium four radial 

 tubes or vessels run out through the substance of the bell into the 

 four larger tentacles, and a circular tube runs about in the margin 

 of the bell and places all the radial tubes in communication. 

 The bell is covered inside and out with ectoderm ; the velum is 

 also ectodermic. and the tentacles and manubrium ai^e covered 

 with it. The inside of the manubrium is endodermic, as are the 

 radial and circular vessels and the inside of the tentacles. The 

 substance of the bell is mainly non-cellular gelatinous matter. 

 The ectoderm at the base of the manubrium is thickened in four 



' See Parker, p. 235, fig 54. 



