i6 



THE HYDROMEDUSAE 



absorption of the tentacles and the diminution in length of an 

 ordinary hydroid which has begun to bud medusoids (Euden- 

 drium). 



A dadylozooid is a hydroid which exhibits modifications corre- 

 lated with its special functions of catching prey. It is elongated, 

 and capable of very active movements, and is either devoid of 

 tentacles (Podocoryne), or provided with short knobs highly 

 charged with nematocysts (Hydractinia, Fig. 22, d). The cnido- 

 phore of Eudendrium racemosum appears to belong to the 

 category of dactylozooids, from which it differs merely in grow- 

 ing from the body of a hydroid, and not from the hydrorhiza 

 (Weismann, 10). 



The MEDUSOID (Fig. 25) is generally conical or hemispherical, 

 in contrast to the next order ; the velum is broad and muscular. 

 The manubrium is generally circular ; the mouth is sometimes sur- 

 rounded by four perradial lobes (Tiara) or four simple or branching 

 capitate "oral tentacles" (Bougainvillea). The marginal tentacles are 

 rarely rudimentary (Amalthea) ; when present they are generally 



hollow ; they number one (Cory- 

 morpha), two (Perigonimus), or 

 six (Clavatella), but are gener- 

 ally only four in number and 

 placed at the ends of the per- 

 radial canals. Interradial ten- 

 tacles may also be present 

 (Podocoryne), or very numerous 

 tentacles arranged in four per- 

 radial groups (Bougainvillea) ; 

 even hundreds may be present 

 (Callitiara), arranged apparently 

 without reference to special 

 radii. Their bases are generally 

 surrounded by a thickened bulb 

 of ectoderm, containing sensory 

 cells and numerous cnidoblasts. 

 The sense organs of the Anthomedusae are ocelli. These consist 

 either of a few pigment cells, hardly grouped into an organ 

 (Euphysa), or of pigment cells grouped into a definite retina, 

 which possesses (Lizzia, Fig. 11) or lacks (Sarsia) a lens. 



They are placed on the bulb of the tentacle, and are generally 

 on its exumbral face, but are on the subumbral face in genera 

 which normally carry their tentacles reflexed (Lizzia). 



The gastric cavity generally lies in the bell, but may be 

 situated at the root of the manubrium (Lar). It often exhibits 

 a prolongation upwards into the substance of the mesogloea of the 

 exumbrella, a relic of the endoderm of the coenosarcal tube by 



FIG. 24. Diagram of Tubularia. b, degenerate 

 medusoids budded from a blastostyle. (After 

 Allman.) 



