v STRUCTURE 313 



and medusa-buds are wanting. The perisarc is therefore a 

 supporting organ or skeleton, not, like our own bones, 

 formed in the interior of the body (endo skeleton) ^ but like 

 the shell of a crayfish or lobster, lying altogether outside 

 the soft parts (exoskeletori). 



As to the mode of formation of the perisarc : we saw 

 that many organisms, such as Sphserella and Amoeba, are 

 able to form a cyst or cell-wall, by secreting or separating 

 from the surface of the protoplasm a succession of layers 

 either of cellulose or of a transparent horn like substance 

 (pp. 244 and 232). But Amoeba and Sphserella are uni- 

 cellular, and are therefore free to form this protective 

 layer at all parts of their surface. The ectoderm-cells of 

 Obelia, on the other hand, are in close contact with their 

 neighbours on all sides and with the mesoglcea at their 

 inner ends, so that it is not surprising to find the secretion 

 of skeletal substance taking place only at their outer ends. 

 As the process takes place simultaneously in adjacent cells, 

 the result is a continuous layer common to the whole 

 ectoderm instead of a capsule to each individual cell. It is 

 to an exoskeletal structure formed in this way, i.e. by the 

 secretion of successive layers from the free faces of adjacent 

 cells, that the name cuticle is in strictness applied in multi- 

 cellular organisms. 



In the blastostyles both mouth and tentacles are absent, 

 the zooid ending distally in a flattened disc : the hydrotheca 

 of a polype is represented by the gonotheca (g.th} which is 

 a cylindrical capsule enclosing the whole structure, but 

 ultimately becoming ruptured at its distal end to allow of 

 the escape of the medusa-buds. These latter are, in the 

 young condition, mere hollow offshoots of the blastostyle : 

 when fully developed they have the appearance of saucers 

 attached by the middle of the convex surface to the blasto- 



