390 HALATRACTUS. 



chord, and when extended presents the appearance of a cylindrical string with ten or twelve little 

 spherules distributed upon it at equal distances. The last of these spherules exactly terminates 

 the string, and is larger than the others. The medusa undoubtedly belongs to the form to 

 which Edward Forbes had given the generic name of Steenstrupia. Of this relation between 

 Steensfrupia and Cori/morplia Forbes himself had a suspicion ; indeed, he expresses a belief that 

 his Steenstrupia rubra will turn out to be the free medusa of Corj/morpha nutans. 



A medusa, which I regard as the adult planoblast of Corymorjjha nutans, was obtained in the 

 open sea, near the spot from which the complete hydroid was dredged. It has been fully described 

 above (p. 211), and from the account there given, it will be seen that the changes undergone by 

 the medusa between the time of its liberation from the trophosome and its attainment of sexual 

 maturity are of little importance. It is especially to be noted that the marginal tentacle always 

 remains solitary. 



When the animal is allowed to assume its natural position in an aquarium, Avith its proximal 

 extremity rooted in the sandy bottom, the stem rises vertically from the point of attachment, while 

 the hydranth usually droops gracefully from the opposite end, its long tentacles forming a beautiful 

 coronal of curved rays round its base, and the short ones forming a dense brush-like cluster 

 round its oral extremity. But there is no part of this beautiful hydroid which tends so forcibly 

 to impress the observer as the gonophores. Nestling at the base of the great coronal of tentacles 

 we may see them in every stage of development, from the nascent bud, in which no medusoid 

 structure can yet be detected, to the fully-formed medusa ready to break away from its restraining 

 stalk, some — as if the life of the hydroid was specially concentrated in these wonderful buds — 

 palpitating in rapid systole and diastole, seemingly eager for their approaching freedom ; while 

 others, less restless, but no less actively engaged in ministering to the welfare of their existence, 

 are casting their long tentacles into the water, marvellous fishing lines, loaded with deadly thread- 

 cells, and sensitive to the slightest touch of the passing prey. 



Corpnor^jha nutans has afforded to me one of the subjects of detailed hydroid study con- 

 tained in the former part of this IMonograph. (See above, p. 208.) 



HALATRACTUS, Allman. 



Name. — From a'Ac, the sea, and arooKroc, a spindle; so called from the form of the 



hydrosome. 



TEOPHOSOME. — Hxdrocaulus surrounded towards its proximal extremity with 

 tubular fleshy processes. Htdranth abruptly distinct from the hydrocaulus ; the 

 proximal set of tentacles in a single verticil, and larger than the distal, which are 

 scattered or subvertillate round the base of a conical hypostome. 



GONOSOME.— Planoblasts sessile, springing from the body of the hydranth 

 between the proximal and distal sets of tentacles ; umbrella at the time of liberation 



