THE HYDROMEDUSAE 



13 



to form a solid crust ; this crust is iu reality composed of separate 



coenoscircal tubes, each surrounded by perisarc. If, instead of the 



perisarc of adjacent tubes becoming adherent or continuous, its 



formation were suspended until the ectoderm of adjacent tubes had 



become confluent, we should 



arrive at the condition presented 



by the central parts of Hydrac- 



tinia (Collcutt, 26); towards the 



edge of the colony this genus 



has the same structure as the 



central parts of Podocoryne ; at 



the growing edge both have a 



loose hvdrorhiza of the usual 



type. 



The tubes of the hydro- 

 caulus are generally distinct, 

 but iu some cases the stem of 

 the colony is " fascicled " or 

 formed of closely apposed or 

 adherent hydrocauli (Euden- 

 driura). Just as this is a modi- 

 fication comparable to the ad- 

 herent hydrorhizal tubes of 

 Podocoryne, so the confluent 

 ectoderm of numerous hydro- 

 cauli in Ceratella (Spencer, 27) 



is comparable to the central hydrorhiza of Hydractinia. A 

 further complication is introduced in the hydrocaulus of Cory- 

 dendrium, owing to the fact that the young buds, instead of 

 breaking through the perisarc and growing outwards as is usual, 

 grow upwards for some distance inside it and surround them- 

 selves by secondary perisarc (Weismann, 10). 



A space generally lies between the ectoderm, and the perisarc 

 of hydrocaulus or hydrorhiza which it secreted ; strands of proto- 

 plasm or elongated ectoderm cells may cross this space. 



The perisarc is rarely absent (Hydra) ; it generally forms a com- 

 plete investment of hydrorhiza and hydrocaulus, and is sometimes 

 prolonged over the body of the hydroid as a sort of hydrotheca (some 

 Bougainvillea), into which the entire hydroid cannot be withdrawn. 

 The perisarc is generally a cuticular secretion of a horny character, 

 but may be formed of adventitious particles held together by a 

 secretion (Perigonimus) ; in both cases the secretion is formed by the 

 activity of the ectoderm cells. A horny perisarc is usually smooth, 

 but may be annulated at the origin of each branch (Cordylophora) 

 or uniformly anmilated throughout its length (Corync). A horny 

 perisarc generally exhibits a concentrically laminated structure, 



Fig. 18.— Colony of Bougainvillea (nai. size) 

 attached to a piece of floating timber. (After 

 AUman.) 



