474 A "Colonial" Hcrbariuui Specimen. [Sess;. 



out in the paper just referred to, currents are set up by the 

 lashing of cilia passing out from cells which have been re- 

 garded as individually bearing a very close affinity with 

 certain infusoria. Living sponges in a vessel soon deprive 

 the water of its oxygen — a desideratum supplied in a re- 

 ciprocal way by growing algae. The reproduction of the 

 sponge is accomplished by " eggs " or gemmules, and by fer- 

 tilised ova which segment and become free-swimming larvse. 



The naked eye can easily discern a few shining beaded 

 threads adhering to the alga. They are the chitinous scaffold- 

 ing of two zoophytes, Obelia and Sertularia. In Obelia there 

 are beautiful transparent cups on ringed stalks, which branch 

 from a common stem. The cups in the fresh state contain a 

 flower-like organism which can protrude itself, and spread out 

 a whorl of waving tentacles to capture food. The mouth is 

 situated in the centre of the whorl, and it communicates with 

 a ciliated canal which runs through the stem, and connects 

 all the zooids (polypites) in a common nutritive system. In 

 certain of the cups there are developed medusa-like buds, 

 which have a nervous system and organs of sense and repro- 

 duction. The buds break away and swim freely in the sea, 

 and give rise ultimately to new stationary colonies. The 

 other hydroid zoophyte present is Sertularia, in which the 

 cups are sessile, and arranged on opposite sides of the stalk. 

 In reproduction the generative buds do not break away and 

 become free as in the Obelia. 



The last items in our " colonial " census are also known as 

 zoophytes, and, like those already described, have often found 

 their way into herbaria as sea-weeds. Covering the rounded 

 stem of the Delesseria is an elegantly reticulated white mesh- 

 work, the protective skeleton of a Polyzoan known as Mem- 

 branipora. The spaces of the meshwork are the cells which 

 separate zooids inhabited. These did not communicate with 

 each other like the zooids of the hydroid zoophytes. They are 

 much higher in organisation ; indeed it surprises one at first 

 sight to find them classed higher in the scale than the shell- 

 fish, and almost at the top of the invertebrate series. The 

 reason becomes apparent when the microscope reveals the fact 

 that they have organs betokening high • development. Each 

 normal zooid has an alimentary canal and a nervous system. 



