v STRUCTURE 313 



resembling the living colony in form except that polypes, 

 blastostyles, and medusa-buds are wanting. The peri- 

 sarc is therefore a supporting organ or skeleton, not, 

 like our own bones, formed in the interior of the body 

 (endoskeletori) , but, like the shell of a crayfish or lobster, 

 lying altogether outside the soft parts (exoskeleton) . 



As to the mode of formation of the perisarc : we 

 saw that many organisms, such as Sphaerella 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 trans- 

 parent horn-like substance (pp. 232 and 244). But 

 Amceba and Sphaerella are unicellular, and are there- 

 fore 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 mesogloea 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 multicellular 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 gono- 

 theca (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 condi- 

 tion, mere hollow offshoots of the blastostyle : when 

 fully developed they have the appearance of saucers 



