256 HYDROIDiE. Part IV. 



and have the same thickness as those of the body (c^). Its cavity does not open 

 outwardly, but, at the base, communicates broadly with that of the convex portion. 

 In this condition the young hydroid is set free, urging its way through the aper- 

 ture of the disk between the cristate tentacles (/ f^)} The tentacles generally 

 trail behind in the egress of the young, and very often, especially when there 

 is but one highly-developed individual present, they occupy the region around the 

 proboscis of the medusoid whilst the body presses against the opposite end. During 

 the struggles of the young to push its way out, the medusoid becomes very much 

 elongated in the direction of its axis. When once fairly freed from its parent, 

 the young hydroid crawls about for a short time upon its long tentacles, and 

 finally turns over and fixes itself by what we have hitherto spoken of as the 

 convex portion of the body (c'). We now recognize the latter as the basal side, 

 or stem, of the individual; and are confirmed in this by the fact that it is cov- 

 ered by a thin, yellowish, glutinous film {Fig. 14", c), which acts as the medium 

 of adherence to whatever the young animal may settle upon for a habitation. 

 This glutinous film, the rudiment of the horny tube which encloses the stem of 

 the hydroid, may be detected, without much difficulty, a short time before the 

 exclusion of the young ; but in order to see it satisfactorily, the hydroid must be 

 cut out of its parent. We are able now to determine what organ the hernia is 

 {Fig. 14, c) which developed from the concave side of the body ; from its jiosition 

 above, and in the centre of the circle of long tentacles, there can be no doubt 

 that it is the proboscis, although it has not yet the proboscidal tentacles. Very 

 soon, however, the end of the proboscis is opened, and around this • opening, or 

 mouth, the buccal tentacles, five, six, or seven in number, develop rapidly. At 

 this stage, the young hydroid appears identical, at least under a low magnifying 

 power, with the young of Thamnocnidia spectabilis, of the same age (PI. XXII. 

 Fig. 15). There are often as many as nine or ten young hydroids, at one time, 

 in a single medusoid (PI. XXIII. Fig. 12), but not all in the same stage of devel- 

 opment; there are those which have been very recently separated from the gran- 

 ular, yellow mass which clings around the proboscis, and have still a spheroidal 

 form {e) ; others with tentacles just budding {Fig. 22, h), some half grown to the 

 age of exclusion {e^ (? e^), and, finally, one or two just leaving the parent. 



When the yellow granular mass has become quite thin, by repeated self-division 

 and the casting off of young hydroids, it may at first sight be very readily 

 mistaken for a second or outer Avail {Fig. 17, a) of the proboscis, but its absence 



' The young resemble so much the small Aca- p. 31, PI. II. Fig. 7), that I am inclined to consider 

 leph-like animal described by Leuckart under the this, also, as the free progeny of the sessile me- 

 name of Pyxidium (Archiv f. Naturg., 1856, Vol. I. dusoid of some European Tubularia. 



