60 



ZOOLOGY. 



sarc) forms around it, tentacles arise around the mouth, 

 finally the stem branches, new Hydroids arise, until a hy- 

 droid community (consisting of trophosomes and gonosomes) 

 is formed, and in the following spring medusa-buds (gono- 

 phores) arise, which become free (medusoids), and thus the 

 reproductive cycle is completed. The developmental his- 

 tory of this Hydroid is a good example of what is called 

 " alternation of generations." 



Budding occurs in the medusa of Sarsia prolifera, in 

 Hybocodon prolifer arid Dysmorphosa fulgurans. Mul- 

 tiplication by fission has been observed in the medusa of 

 Stomobrachium mirabile. The pendent stomach was seen 

 by Kolliker to divide in two, becoming doubled, which act 

 was followed by a vertical division of the umbrella, separat- 

 ing the animal into two independent halves. These again 

 subdivided, and Kolliker thinks this process went on still 

 further. Haeckel has found in cutting off a portion of the 

 edges of the umbrella of certain Thaumantice, that the frag- 

 ment in a few days became a complete medusa. 

 In the Tubularian Hydroids (Tubularia, Hylocodon, Co- 

 rymorpha, Monocaulus, etc., Fig. 41), 

 the mode of reproduction is peculiar. 

 From the medusa-buds (sporosac) is set 

 free an embryo (actinula), which swims 

 about or creeps on its tentacles, mouth 

 downward. It then attaches itself by a 

 disk-like expansion of the posterior end, 

 which forms a stem until the original 

 Tubularia form is attained. 



A gigantic Monocaulus having sessile 

 ovisacs, measuring seven feet four inches 

 in height, and provided with a crown of 

 tentacles nine inches across from tip to 

 tip of the expanded, non-retractile ten- 

 tacles, was dredged by the Challenger 

 Expedition at the depth of four miles. 

 Ailman suggests that such a deep-sea Hydroid could not, on 

 account of the darkness and pressure of the water at such a 

 great depth, produce free-swimming medusae. In Tiaropsis 



41. Monocaulus pen- 

 tlus. After Agassiz. 



