258 HYDROID^. Part IV. 



pushed up from beneath. This forms a cone {Fig. 6, d) with ca hollow ulterior 

 (c^) which is in direct and broadly open communication with the cavity [c) of 

 the pedicel. Although this cone is at no time open at the end, and, consequently, 

 food is never taken in through its instrumentality, yet it is in every respect, 

 homologically, the proboscis of the medusoid. As the proboscis lengthens, its wall 

 thickens, until, by the time the tip of the former has nearly reached the end of 

 the medusoid, the latter {Fig. 7% d) has become as thick as the inner Avail {b^) 

 of the pedicel. From this time the germ-basis, or the spermatic mass, ceases to 

 grow as fast as the cavity of the disk enlarges. This at first makes itself evident 

 near the end of the medusoid, where there is a space, which the germ-basis 

 {Fig. 8, /) does not fill up. There is as yet only a single wall {h) which protects 

 the space from the surrounding medium, the edge {e) of the cup-shaped inner 

 wall {c) not being closed over. Simultaneous with this feature there appears 

 another quite as noteworthy, in the comparatively much diminished size, and in 

 the change of shape of the proboscis {d). Whereas heretofore it has occupied a 

 very broad basis, nearly equalling the breadth of the medusoid, and also has filled 

 a great portion of the cavity of the disk {Figs. 6, 7, and 7") ; it now projects into 

 the axis of the disk, in the form of a slender cylindrical pillar {Fig. 8, d). Soon 

 after this, both in the female {Fig. 9) and male {Fig. 15), the proboscis {d^) forces 

 its way through the germ-basis, or spermatic mass, and even projects through the 

 recently formed opening {Figs. 9, /, and 15, e) of the disk. We have not made 

 sure of the fact whether the inner wall closes over by uniting the edge of the 

 cup mentioned above, or not; but in all probability it does not so happen, because 

 the aperture in the disk is formed immediately after the vacant space is left between 

 the outer wall and the germ-basis. In the male no further development, excepting 

 an increase in bulk, takes place ; but in the female a remarkable set of tentacles 

 {Fig. 10, /) are formed. The time of their development is not always contempo- 

 raneous with certain other phases ; sometimes they are largely developed before 

 the germ-basis {Fig. 10, e) has begun to segment, and at others they have not 

 appeared, although some of the young hydroids {Fig. 11, e) have already separated 

 from their basis {g). The mode of formation of the tentacles is very simjile : 

 around the opening of the disk, the double walls become plicated in the direction 

 of the axis of the medusoid, and these folds, varying from five or six to ten in 

 number {Fig. 13, /), project outwardly in the form of low thin crests, the planes 

 of which trend toward the axis of the medusoid. At first they are about as 

 long, at the base, as high, and run out to a blunt point so as to form a triangle. 

 After this, the fold extends toward the pedicel of the medusoid, and reaches 

 sometimes along half the length of the disk, at the same time diminishing in 

 height until finally it runs off into the walls from which it originates ; and the 



